MICHEI   LOUTFALLAH 


THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

LOS  ANGELES 


TESTS  AND  STUDIES 


OF  THE 


OCULAR  MUSCLES 


ERNEST  E.  MADDOX,  M.D.,  F.R.C.S.,  Ed. 

Ophthalmic  Surgeon  to  Royal   Victoria  Hospital,  Bournemouth  ;  formerly  Syme 
Surgical  Fellow,  Edinburgh  University 


WITH  110  ILLUSTRATIONS 


THIRD  EDITION 
Specially  revised  and  enlarged  by  the  author 


PUBLISHED    BY 

THE  KEYSTONE  PUBLISHING  CO. 

BOURSE  BLDG.,  PHILADELPHIA,  U.S.A. 

1907 


• 


PREFACE  TO  THE  THIRD  EDITION 


That  a  second  edition  of  this  book  should  be  asked  for  affords 
pleasant  compensation  for  the  time  and  thought  it  cost. 

In  the  choice  of  terminology,  I  have  carefully  refrained  from 
the  dislodgment  of  ancient  landmarks.  Thus  the  words  "adduction" 
and  "abduction"  will  be  found  retained  in  their  time-honored 
sense — a  sense  which  is  stamped  on  the  very  name  of  "  abducens" 
muscle  itself,  and  the  dislocation  of  which  would  make  the  classics 
of  Graefe  and  Bonders,  Helmholtz  and  Mauthner  less  intelligible. 

The  announcement,  by  one  author  and  another,  of  supposed 
errors  in  the  treatment  of  Listing's  law  by  Bonders  and  by  Helm- 
holtz have  compelled  me,  even  in  these  uncontroversial  pages,  to 
enter  more  fully  into  the  laws  of  the  parallel  motions  of  the  eyes 
than  their  clinical  importance  would  perhaps  otherwise  warrant. 
The  purely  clinical  reader  can,  if  he  prefer,  pass  over  the  section 
which  shows  these  discoveries  to  be  groundless. 

My  thanks  are  due  to  Br.  Asher,  of  Leipsig,  tor  several 
suggestions  from  the  German  edition,  one  of  which  has  led  me  to 
add  a  chapter  on  Nystagmus. 

E.  E.  M. 


CONTENTS 


CHAPTER  I  PAOK 

THE  GLOBE  AND  ITS  SOCKET 15 

The  Ball  of  the  Eye— Ocular  Muscles— Normal  Motions— Orbits- 
Tenon's  Fascia — Check  Ligaments — Their  Functions — Internal  Cap- 
sule— Tenon's  Space — Bearing  of  Check  Ligaments  on  Tenotomy. 

CHAPTER  II 
THE  OCULAR  MOTIONS 34 

Translations  of  the  Globe — Listing's  Plane — False  Torsion — 
Bonder's  Law — Listing's  Law — Helmholtz's  Plane  of  Reference — 
Donder's  Plane  of  Reference — Index  of  Torsion — Use  of  False 
Torsion — Azimuth  and  Altitude. 

CHAPTER  III 
INDIVIDUAL  OCULAR  MUSCLES 52 

Law  of  Innervation — Description  of  the  Recli — Spiral  of  Insertions 
— Description  of  Obliques — Subsidiary  Functions — Medial  Origin 
of  Muscles — Superductors  and  Subductors — Lines  of  Force — 
Muscular  Planes — Axes  of  Rotation — Paralytic  Equilibrium — Con- 
secutive Deviation — Arc  of  Contact — Ophthalmotropes — Landolt's 
Ball— Tilted  Axes. 

CHAPTER  IV 
ASSOCIATED  MUSCLES  IN  A  SINGLE  EYE 68 

Isolated  Contraction  of  some  Muscles  Unknown — Superducting 
Muscles  for  Example — Motion  from  Secondary  Positions — Compo- 
sition of  Rotations — Dynamics — Moments — Resolution  of  Rotations 
—Co-ordination — Vertical  Purchase  of  Recti  and  Obliques  altered 
by  Adduction  and  Abduction — Purchase  of  same  Muscles  about 
Vertical  Axis — Paralytic  Exophthalmos — Model  with  Tilted  Axes — 
Paralytic  Semi-orbits. 

7 


S  Tests  and  Studies  of  the  Ocular  Muscles 

CHAPTER  V  PAGB 

CONJUGATION  OF  THE  Two  EYES 79 

True  Associates — Spasm  of  Single  Muscles — Conjugate  Innerva- 
tions — Conjugate  Paralyses — Convergence — Confirmation  of  Rules 
of  Conjugation — Convergence  and  Accommodation — Effort  and 
Work — Exophoria  in  Oblique  Vision — Latent  Deviations. 

CHAPTER  VI 
MOTOR  FACULTIES g€ 

Point  of  Fixation — Direct  and  Indirect  Vision — Fixation-reflex — 
Persistent  Fixation — Binocular  Fixation — Projection — Correspond- 
ing Points — Physiological  Diplopia — Suppression  of  Images — 
Origin  of  Projection — Fusion — Power  of  Overcoming  Prisms — 
Breadth  of  Fusion  Power — Monocular  Perception  of  Distance — 
Stereoscopic  Vision  of  Relief — Hering's  Drop  Test. 

CHAPTER  VII 
STRABISMUS 115 

Definition — Chief  Division — Strabismus  Convergens — Accommo- 
dative —  Non-accommodative  —  Predisposing  Causes  —  Congenital 
Amblyopia— Nervous  Element  of  Strabismus— Imperfect  Central 
Fixation — Strabismus  Incongruus — Recovery  of  Lost  Faculties — 
Extension  of  Partially  Preserved  Faculties — Strabismus  Convergens 
Myopicus — Alternating  Squint — Unilateral  Squint — Operations — 
Strabismus  Divergens — Refractive  After-treatment  for  Squints — 
Javal's  Course  of  Treatment — Natural  Cure  of  Squint — Evidences 
of  Squint — Secondary  Deviation — Fallacy  from  Anisometropia — 
Apparent  Squint — Intrinsic  Aberrations  of  Eyeball — Linear  Stra- 
bismometry — Hirschberg's  Method — Perimeter  Method — Charpen- 
tier's  Method — The  Tangent  Strabismometer — Subjective  Strabis- 
mometry — Paralytic  Strabismus. 

CHAPTER  VIII 
OCULAR   PARALYSES 151 

Symptoms — Order  of  Examination — Diplopia — Aphorisms,  Incor- 
rect and  Correct — Complications— Diagnostic  Procedure — Narrow- 
ing Circles — Dextral  and  Laeval — Torsional  Purchase  and  Vertical 
Purchase — Brief  Summary — Confirmation  of  the  Diagnosis — 
Measured  Charts — To  Read  a  Simple  Chart — To  Read  a  Multiple 
Chart — Independent  Diplopise — Clinical  Classification  of  Muscles. 

CHAPTER  IX 
OCULAR  PARALYSES  {Continued) « 165 

Optical  Illusion — How  to  transfer  Charts  to  Opposite  Eyes — Paraly- 
sis of  Right  External  Rectus — Paralysis  of  Left  External  Rectus — 


Contents  9 

Qualifications  in  Paralysis  of  the  External  Recti — Paralysis  of  Right 
Internal  Rectus — Paralysis  of  Left  Internal  Rectus — Qualifications 
in  Paralysis  of  the  Internal  Recti — Paralysis  of  Right  Superior 
Rectus — Paralysis  of  Left  Superior  Rectus — Paralysis  of  Right 
Inferior  Rectus — Paralysis  of  Left  Inferior  Rectus — Paralysis  of 
Right  Superior  Oblique— Paralysis  of  Left  Superior  Oblique — 
Paralysis  of  Right  Inferior  Oblique—  Paralysis  of  Left  Inferior 
Oblique — Paralysis  of  the  Third  Nerve — Test  for  Fourth  Nerve — 
Measurement  of  Ocular  Paralyses — Conjugate  Paralyses — Tests  for 
them — Tropometer — Precise  Tests  for  Convergence — Mnemonics 
for  Ocular  Paralyses — Dangers  of  Mnemonics — Werner's  Mnemonic. 

CHAPTER  X 
NYSTAGMUS 192 

Definition — Rapidity — Apparent  Motion — Examination  of — Etiology 
— Nature  of  the  Excursions — Treatment  of  Nystagmus — Curiosities. 

CHAPTER  XI 
OPHTHALMOSCOPIC  CORNEAL  IMAGES 198 

Precautions — Fixation  Position  of  the  Corneal  Reflection — Apparent 
Squint — Refraction  Surmisable — The  Reflection  in  Cataract  and 
Iridectomy — Tests  for  Monocular  Blindness — Alternation — Conco- 
mitancy — Test  for  Binocular  Fixation — Hirschberg's  Rule  for 
Squint — Priestley  Smith's  Mode  of  Strabismometry — Different 
Points  of  View — Error  of  Approximation — Photography  of  Muscu- 
lar Anomalies — Author's  Camera — Recording  Reflections. 

CHAPTER  XII 
HETEROPHORIA       '.    .    , 213 

Chief  Divisions — Dissociation  of  the  Eyes — Physiological  Hetero- 
phoria — Direction  of  Deviation — Hyperphoria — Objective  and  Sub- 
jective Tests — Prism  Tests — Glass  Rod  Test  and  its  Allies — Two 
Species  of  Cylinder — Manufacture — Mode  of  Use — Measurement  of 
Latent  Deviations — Rare  Anomaly — Previous  Tangent  Scales — 
Advantages  of  the  Tangent  Scale — Which  Eye  Fixes  ? — Other  Uses 
of  the  Scale — Test  for  Concomitancy — Trial  Test  for  Reliability — 
Heterophoria  in  Near  Vision — Mode  of  Use — Its  Meaning — Physio- 
logical Exophoria — Prism  Diopters  in  Near-Vision  Tests — Inter- 
mediate Scales — Tests  for  Breadth  of  Fusion — Correction  of  Hyper- 
phoria— Relative  Importance  of  Correction — Concomitancy  vs. 
Paresis — Horizontal  Breadth  of  Fusion — Orthoptic  Training — 
Simple  Rules  for  Heterophoria — Rules  for  Decentering — :Prism 
Diopters — Direction  of  Decentering — Example — Prism  Diopters — 
Operative  Interference — Graduated  Tenotomy — Marginal  Tenotomy. 


io  Tests  and  Studies  of  the  Ocular  Muscles 

CHAPTER  XIII  PAGE 

CVCLOPHORIA 236 

Its  Detection  and  Clinical  Measurement — Its  Exact  Measurement — 
Paretic  Cyclophoria — Non-paretic  Cyclophoria — Explanation  of 
Leaning  Image — Rule  for  Rod  Test — Cyclophoria  in  Near  Vision — 
Oblique  Astigmatism — Cyclophorometers — Optomyometer — Clino- 
scope — Volkmann's  Apparatus — Meissner's  Test — Depression  of 
the  Visual  Plane — True  Primary  Position  in  Distant  and  in  Near 
Vision — Le  Conte's  Confirmation — Savage's  Test  Compared — 
Eaton's  Apparatus — Formula  for  Meissner's  Test. 

CHAPTER  XIV 
THE  EYE  IN  DARKNESS 250 

The  Visual  Camera — Deviation  in  Near  Vision — Sense  of  Projec- 
tion— Effect  of  Attention  on  the  Desire  for  Fusion — Speed  of  the 
Exophoria — Laws  of  Conjugation  Illustrated — False  Fusion — 
Diluted  Fusion. 


LIST  OF  ILLUSTRATIONS 


1-2 — Axes,  Poles  and  Equator 17 

3-4 — Tenon's  Fascia 19,    21 

5-7 — Check  Ligaments 22,    23 

8 — Horizontal  Section  of  the  Globe  and  its  Membranes  ....  24 

9-10 — Diagrams  showing  effect  of  Tenotomy 26,    27 

ii — Check  Ligaments 29 

12 — Vertical  longitudinal  section  of  Eyeball  and  its  adnexa     .    .  30 

13 — View  of  under-surface  of  the  Eyeball 31 

14 — Author's  Torsion  Calculator       ....       43 

15 — Author's  Design  for  solving  False  Torsion 47 

16— Diagram  showing  how  the  Eye  reaches  a  new  position  by  the 

shortest  route 48 

17-21 — Diagrams  illustrating  Torsion 49,    50 

22 — Muscular  Planes  and  Axes 59 

23 — Landolt's  Ophthalmotrope 63 

24 — Anderson  Stuart's  Model  of  Ocular  Muscles 64 

25— Landolt's  Ball 65 

26 — Tilting  of  the  Axes 66 

27 — Helmholtz's  Diagram  for  motion  from  one  secondary  posi- 
tion to  another 69 

28 — Composition  of  Rotations 71 

29— Composition  of  Muscular  Forces 72 

30 — Planes  of  Corneal  Orbits 73 

31— Model  with  Tilted  Axes .  76 

32 — Diagram  illustrating  Conjugation 87 

33-36 — Diagrams  illustrating  Convergence  and  Accommodation  .    .  88,    92 

37-38 — Figures  for  the  Field  of  Fixation 101 

39 — Diagram  illustrating  Mis-projection 106 

40 — Proximal  and  Distal  Diplopiae in 

41-43 — Stereoscopes HI,  112,  113 

44 — Home-made  form  of  Hering's  Drop  Test 114 

45 — Ophthalmoscopic  Corneal  Reflections  in  Normal  Eyes  .    .   .  137 

46-47 — Diagrams    illustrating    Perimetric    Methods    of    Measuring 

Squint 140 

48 — Diagram  to  illustrate  "Spherical  Aberration  " 141 

49-51 — First  and  Second  Steps  in  Tangent  Strabismometry  .    .  142,  143,  144 

52 — Landolt's  Dynamometer 154 

53 — Schweigger's  Hand  Perimeter 155 

54 — Dextral  and  Laeval  Muscles 162 

55 — Mnemonic  Attitude  for  Muscular  Planes 162 

ll 


1 2  Tests  and  Studies  of  the  Ocular  Muscles 


56 — Method  for  Inscribing  Charts 165 

57-70 — Charts  of  Ocular  Paralyses  .    .  169,  170,  172,  173,  174,  175,  176,  177 

178,  179,  181,  182 

71 — Tangent  Scales 183 

72 — Specimen  of  Chart  Entry 183 

73— Kick's  Motor  Chart 186 

74 — Stevens'  Tropometer 190 

78 — Corneal  Reflections  in  Normal  Eyes 199 

79 — Diagram  showing  Obliquity  of  Visual  Axis  with  reference 

to  Geometrical  Axis 200 

80-81 — Diagrams  showing  Symmetry  and  Asymmetry  of  Normal 

Eyes 203 

82 — Unsymmetrical  Angles  Gamma 204 

83 — Priestley  Smith's  Tape  Method 206 

84 — The  Caustic  Curve  of  a  Convex  Mirror 207 

85 — Analysis  of  Corneal  Reflection 208 

86 — Photograph  of  Ascending  Convergent  Squint 209 

87 — Author's  Camera  for  photographing  Reflections 209 

88 — Photograph  of  over-correction,  immediately  after  advance- 
ment   211 

89 — Chart  of  Congenital  Defect  of  Right  Superior  Rectus    .   .  211 

90-91 — Author's  Double  Prism 217 

92 — Prisms  of  Stevens'  Phorometer 218 

93— First  form  of  the  Glass-rod  Test 220 

94 — Diagram  showing  Use  of  Rod  Test  „ 221 

95 — Dioptric  Tape  Measure 222 

96 — Author's  Tangent  Scales      223 

97-98 — Trial-Frame  for  Use  of  Prisms 228,  229 

99— Diagram  showing  Prismatic  Effect  of  Decentering      ...  232 

100 — Savage's  Test  .    .       238 

100% — Illustrating  Oblique  Astigmatism 239 

101 — Optomyometer  of  the  Geneva  Optical  Company     ....  241 

102 — Stevens'  Clinoscope 242 

103-105 — Haploscopic  Figures 243,  244 

106 — Eaton's  Apparatus  for  Meissner's  Test 246 

107-108 — Author's  Plan  for  solving  Meissner-Torsion 248 

109 — The  Visual  Camera 251 

no — Camera  for  Testing  Projection 254 


TESTS  AND  STUDIES 

OF  THE 

OCULAR  MUSCLES 


TESTS  AND  STUDIES  OF  THE  OCULAR  MUSCLES 


CHAPTER  I 


The  Globe  and  Its  Socket 

The  motions  of  the  eyes  are  notable  for  their  combination  of 
silence,  swiftness  and  precision. 

The  silence  of  the  eye,  or,  at  least,  the  absence  of  audible 
sound,  is  all  the  more  remarkable  because  of  the  proximity  of  the 
organ  of  hearing  and  the  ready  conduction  of  sound  by  bone. 

The  swiftness  of  the  eyeball  itself  is  not,  perhaps,  greater 
than  that  of  adept  fingers,  nor  is  it  desirable  that  it  should 
be  in  the  interest  of  its  delicate  contents;  yet  the  act  of  winking 
or  "twinkling  of  the  eye"  has  always  been  accepted  by  com- 
mon consent  as  the  briefest  measure  of  time  expressible  by  the 
human  body. 

The  precision  of  the  ocular  movements,  together  with  the 
perfect  co-ordination  of  the  two  eyes,  is  the  most  important  virtue 
of  the  three,  and  is  evidenced  in  thousands  of  ways  every  day. 
One  example  only  need  be  given,  namely,  that  in  watching  a  small 
moving  object  in  the  distance,  such  as  a  bird  a  mile  away,  it  is  seen 
single  instead  of  double,  which  could  not  be  unless  both  eyes 
followed  the  object  with  the  keenest  exactness. 

Akin  to  this  we  may  mention  the  steadiness  of  the  eyeball  in 
observing  a  fixed  point ;  a  steadiness,  however,  which  is  not 
inherent  in  the  ocular  muscles,  but  which  is  maintained  by  an 
exquisite  "visual  reflex"  mechanism. 

When  the  eyelids  are  closed  the  globes  are  in  almost  perpetual 
motion,  as  any  reader  may  verify  by  laying  the  tips  of  his  fore- 
fingers over  the  closed  upper  lids:  moreover,  if  one  eye  be  covered 
while  the  other  is  observing  with  comparative  steadiness  a  fixed 
point,  the  covered  eye  does  not  share  the  steadiness  of  its  fellow, 
but  wavers  slowly  from  side  to  side.  This  is  easily  demonstrated 
by  the  author's  "visual  camera"  (Chapter  XIV),  which  detects 
the  movements  of  an  eye  placed  in  the  dark. 

H 


1 6  Tests  and  Studies  of  the  Ocular  Muscles 

Even  in  the  light,  an  eye  is  unsteady  unless  occupied  with  a 
fixed  object,  as  when,  for  instance,  it  only  sees  a  false  image  of  an 
object,  the  true  image  of  which  is  seen  by  the  other  eye. 

The  absolute  steadiness  of  the  eye  during  the  study  of  minute 
objects,  is  entirely  beyond  our  voluntary  control,  and  I  think  we 
may  fairly  describe  the  parts  played  by  volition  and  reflex  action 
respectively,  when  we  say  that  the  former  directs  the  eye,  and  the 
latter  steadies  it.  It  is  true  that  in  daily  life  the  point  of  fixation  is 
constantly  on  the  move,  but  then  it  does  not  move  in  a  wavering 
way,  but  purposefully,  and  in  looking  at  an  object  it  flits,  as  it  were, 
from  one  salient  point  to  another,  dwelling  upon  each  long  enough 
to  let  the  mind  grasp  the  new  picture  presented  each  time. 
Lamare's  ingenious  plan  of  making  the  movements  of  the  eye 
audible  by  a  kind  of  binaural  stethoscope  attached  by  a  point  to 
the  upper  lid,  showed  that  four  or  five  slight  movements  take  place 
during  the  reading  of  one  line,  and  a  greater  movement  when  we 
begin  to  read  a  new  line. 

Under  ordinary  conditions  we  can  turn  our  eyes  at  pleasure 
from  one  object  to  another,  but  there  is  a  peculiar  pathological 
state  in  which  this  faculty  fails,  and  in  which  this  visual  reflex 
appears  to  gain  the  upper  hand,  so  that  the  eyes  can  with  difficulty 
be  made  to  look  away  from  the  object  last  looked  at.  To  this 
subject  we  shall  recur  later  on. 

The  Ball  Of  the  Eye- — When  we  consider  the  spheroidal  shape  of 
the  eyeball,  and  the  character  of  its  motions,  we  need  not  wonder  that 
astronomical  language  has  been  so  freely  drawn  upon  for  their  descrip- 
tion. Thus  we  speak  of  the  globe  moving  in  its  orbit  (metaphorically 
like  a  planet),  and  distinguish  its  axis,  poles,  meridians  and  equator. 

The  anterior  pole  is  the  mid-point  of  the  cornea  in  front:  the 
posterior  pole  the  mid-point  of  the  sclerotic  behind  (as  in  Fig.  i). 

The  axis  of  the  eye,  often  called  the  "optic  axis,"  extends 
between  these  poles.  ' 

The  equator  is  a  circle  or  belt  of  the  globe  midway  between 
the  two  poles.*  (Fig-  2.)  •> 

The  meridians  are  circles,  each  of  which  passes  through  both 
poles  so  as  to  have  the  axis  of  the  eye  for  their  common  diameter. 

In  the  study  of  the  ocular  motions  we  assume  the  eyeball  to  be 

*  This  definition  of  the  equator  is  an  anatomical  one.  Physiologically,  its  axis  coin- 
cides with  the  visual  line,  if  we  think  of  vision  ;  or  with  the  fixation  line  if'we  are  occupied 
with  the  nrular  motions.  Since  the  eyeball  is  not  a  geometrically  true  body,  it  is  customary 
t"  disregard  the  little  discrepancies  between  the  position  of  the  anatomical  equator  aud  those 
of  the  visual  and  fixation  lines. 


The  Globe  and  Its  Socket 


Fig.  I 


spherical,  though  it  is  not  strictly  so,  but  flattened  Irom  before 
backwards,  more  like  an  "oblate  spheroid,"  interrupted  by 
the  prominence  of  the  cornea  in  front,  which  has  a  stronger 
curvature. 

Ocular  Muscles. — Each 
eyeball  receives  the  inser- 
tions of  six  muscles,  namely, 
four  recti  and  two  obliques. 
The  recti  have  an  almost 
common  origin  around  the 
optic  foramen  (embracing 
the  optic  nerve  at  its  en- 
trance into  the  orbit)  and 
course  forwards,  diverging  as  they  go  to  embrace  the  globe 
for  a  short  distance  before  reaching  their  insertions. 

The  superior  and  inferior  oblique  muscles  act  upon  the  eye 
from  the  upper  and  lower  corners  respectively  of  the  inner  v/all  of 
the  orbital  outlet. 

The  motions  of  the  globe  take  place  unerringly  under  the 
guidance  of  these  six  delicately-proportioned  muscles,  the 
importance  of  whose  contribution  to  our  daily  comfort  is  not 
realized  till  one  of  them  is  disabled  from  any  cause,  and  we 
see  double. 

Normal  Motions. — The  eye  is  so  suspended  in  position  that  its 
ordinary  movements  are  limited  to  those  of  rotation,  no  appreciable 
translation  being  possible.  It  is  true  that 
certain  animals  possess  a  "retractor  muscle 
of  the  globe,"  capable  of  drawing  the  eye 
deeper  into  the  recess  of  the  orbit,  but  in 
man,  exophthalmos  and  enophthalmos  are 
only  known  as  pathological  conditions,  due, 
in  part,  to  such  causes  as  variations  in  the 
size  of  the  palpebral  aperture,  varying  pres- 
sure ot  the  lids,  varying  tone  of  the  extra- 
ocular  muscles  ;  turgescence  or  spasm  of  the 
retro-ocular  blood  vessels,  spasm  and  relaxation  of  the  unstriped 
"orbital  muscle''  of  Muller,  which  spans  the  spheno-maxillary 
fissure  ;  and  possibly  also  to  contraction  or  relaxation  of  the 
unstriped  muscular  fibres  described  by  Sappey  as  existing  in  the 
internal  and  external  check  ligaments  near  their  orbital  insertion. 


Fig. 


iS  Tests  and  Studies  of  the  Ocular  Muscles 

It  is  probable  that  there  occur,   even    in  health,   slight  unnoticed 
physiological  variations  in  the  prominence  of  the  eyes. 

Orbits. — The  orbits  are  two  deep  conical  excavations  in  the 
skull,  the  anatomy  of  which  is  too  well  known  to  need  description. 

At  the  apex  of  the  cone  are  two  apertures,  the  optic  foramen 
and  the  sphenoidal  fissure,  the  former  transmitting  the  optic  nerve 
and  the  ophthalmic  artery,  the  latter  all  the  other  nerves  but  one, 
and  the  ophthalmic  veins.  The  inner  walls  of  the  two  orbits  are 
almost  parallel  to  each  other,  but  the  outer  walls  slope  outwards 
so  strongly  that  the  axes  of  the  two  orbits  (represented  by  imaginary 
lines  from  the  apices  to  the  centers  of  the  orbital  outlets)  diverge 
from  each  other  by  from  24°  to  30°.  The  conical  shape  of  the 
orbit  is  to  accommodate  the  cone  of  muscles,  and  its  apparently 
superabundant  capacity  is  to  permit  the  globe  to  be  sufficiently 
packed  in  with  orbital  fat  which  plays  a  very  important  part  in  the 
formation  of  its  socket. 

The  orbital  outlet  is  narrowed  a  little  by  the  incurving  of  its 
upper  and  outer  margins,  and  its  outer  margin  is  considerably 
posterior  to  its  inner.  From  a  series  of  measurements  which.  I  have 
made,  the  outer  margin  of  the  bony  orbit  appears  to  be,  on  an 
average,  about  22  mm.  behind  the  root  of  the  nose,  12  mm.  behind 
the  anterior  ridge*  of  the  lachrymal  groove,  and  even  7  mm.  behind 
the  depression  for  the  trochlea  of  the  superior  oblique.  A  needle 
run  transversely  inwards  athwart  the  outer  margin  of  the  orbit 
would,  it  is  said,  pierce  the  center  of  an  average  eyeball.  But 
great  differences  exist.  The  orbit  is  i^  inches  deep,  while  its 
outlet  is  i)^  inches  broad  and  i^  inches  high. 

It  is  evident  that  the  large  size  and  conical  shape  of  the  orbit 
would  make  it  by  itself  a  most  unsuitable  socket  for  the  eye  to 
work  in  ;  but,  as  a  matter  of  fact,  the  eyeball  comes  nowhere  in 
contact  with  it,  and  it  may  be  regarded  rather  as  a  strong  scaffolding 
for  the  real  socket,  as  well  as  a  storehouse  for  the  orbital  contents. 
The  real  socket  is  the  capsule  of  Tenon,  in  conjunction  with  its 
supporting  bed  of  orbital  fat,  supplemented  by  the  concave  surface 
of  the  eyelids  in  front. 

Tenon's  Fascia. — All  the  structures  contained  within  the  orbit 
are  invested  by  sheaths  derived  from  one  and  the  same  aponeurosis. 
The  cornea,  which,  at  first  sight,  appears  to  be  an  exception  to  this 

*  The  ridge  referred  to  is  the  front  edge  of  the  groove  formed  by  the  superiol 
maxilla  and  lachrymal  bone. 


The  Globe  and  Its  Socket  19 

rule,  is  not,  of  course,  strictly  within  the  orbit.  This  "orbital 
fascia  "  is  in  some  places  or  parts  of  it  exquisitely  differentiated  to 
suit  the  requirements  of  the  ocular  motions,  and  where  it  surrounds 
the  sclerotic  forms  the  outer  layer  of  Tenon's  capsule,  or,  in  other 
words,  the  external  capsule  of  the  eye.  It  is  lined  by  a  delicate 
internal  capsule,  which  is  of  a  different  nature,  being  regarded  by 
some  as  the  serous  membrane  (/.  e. ,  the  pericardium  or  pleura)  of 
the  eyeball. 

The  common  aponeurosis  of  the  orbit  extends  from  one  struc- 
ture to  another,  splitting  to  encapsule  each,  but  it  is  convenient  to 
commence  its  study  by  distinguishing  that  part  of  it  which  is 
specially  in  relation  to  the  ocular  muscles. 

We  have  already  seen  that  the  orbit  contains  a  group  of 
muscles  which  spring  from  the  circumference  of  the  optic  foramen, 
and  separate  as  they  proceed  forwards,  so  as  to  form  a  cone. 

Ensheathing  this  muscular  cone  there  is  a  fascial  cone,  which 
extends  from  muscle  to  muscle,  splitting  to  invest  each  with  a 
fibrous  sheath,  and  sending  off  layers  here  and  there  to  enclose 
lobules  of  fat,  vessels  and  nerves. 
This  cone  of  fascia  is  attached  at  the 
apex  of  the  orbit  to  the  periosteum 
round  the  optic  foramen,  and  widens 
as  it  advances  till  it  gains  the  orbital 
outlet,  to  be  rigidly  attached  to  the 
periosteum  all  round  the  margin. 

There  is,  therefore  (as  shown  at 
Fig-   3  ),   a  kind    of    cone  of  fascia 

Fig.  3 

within  a  cone  of  bone,  with  this  dif- 
ference between  them,  that  while  the  bony  cone  contracts  at  its 
brim,  the  fascial  cone  expands  at  its  brim,  so  that  an  interval  exists 
between  the  two  which  is  filled  up  with  the  peri-ocular  fat,  the  extra- 
muscular  fat,  the  lachrymal  gland,  etc.  Fig.  3  shows  very  clearly 
how  the  eyeball  is  suspended  in  this  cone,  from  above,  and  from  all 
sides  as  well  as  from  below  ;  so  that  the  part  beneath  the  eyeball, 
which  partly  supports  it  as  on  a  hammock,  receives  too  much  credit 
by  the  name  hitherto  given  to  it,  of  "the  suspensory  ligament  of 
the  eyeball." 

The  fascial  cone  lodges  the  eyeball  in  front,  and  the  retro-bulbar 
fat  behind.  It  is  divided  into  two  compartments — an  anterior  one 
for  the  eyeball,  and  a  posterior  one  for  the  retro-bulbar  fat — by  a 


2o  Tests  and  Studies  of  the  Ocular  Muscles 

hemispherical  aponeurotic  septum  {P.  E.  C.  in  Fig.  4),  which 
adapts  itself  to  the  posterior  hemisphere  of  the  eyeball.  This 
septum  is  given  off  from  the  fascial  cone  just  opposite  the  equator 
of  the  globe  all  around,  and  from  the  same  line  of  origin  springs  a 
companion  membrane  (A.  E.  C. )  which  passes  forward  over  the 
anterior  hemisphere,  investing  it  pretty  closely  as  far  as  the  margin 
of  the  cornea,  where  it  becomes  attached. 

These  t\vo  eye-investing  membranes  are  regarded  as  forming 
one  capsule,  known  as  Tenon's  external  capsule.  It  sends  pro- 
longations backwards  in  the  form  of  a  sheath  for  the  optic  nerve 
(separated  from  it  by  the  supra-vaginal  lymph  space)  and  for  the 
various  vessels  and  nerves  which  enter  the  eye,  and  though  nothing 
more  than  a  part  of  the  common  aponeurosis  of  the  orbit,  is  endowed 
with  remarkable  elasticity. 

Since  on  reaching  the  edge  of  each  muscle  the  fascial  cone 
splits  into  two  layers,  one  to  cover  the  orbital  surface  of  the  muscle 
and  the  other  to  cover  the  ocular  surface,  we  find  on  studying  a 
longitudinal  section  of  a  muscle  that  we  have  to  take  account  of 
these  two  layers,  as  in  Fig.  4.  The  deep  layer  (D. )  becomes  con- 
tinuous at  the  equator  of  the  eye  (/  C.  L.}  with  the  posterior  hemi- 
sphere of  Tenon's  capsule  (P.  E.  C. ),  so  that  from  thence  forwards  the 
deep  surface  of  the  muscle  and  its  tendon  have  no  fascial  investment. 

When  we  consider  the  orbital  layer  of  each  muscle-sheath  we 
find  the  case  is  not  so  simple.  As  it  approaches  the  neighborhood 
of  the  globe  it  thickens,  and  becomes  more  closely  attached  to  the 
muscle  itself,  till  opposite  the  equator  the  attachment  reaches  its 
maximum  ;  after  that  it  quits  the  muscle  altogether,  though  not 
until  it  has  sent  off  a  prolongation  forward  over  the  tendon  to 
contribute  to  the  anterior  portion  of  the  globe's  investment 
(A.  E.  C. ),  and  proceeds  in  the  form  of  a  thick  band  (Ext.  C.  L. ) 
to  the  orbital  margin. 

Check  Ligaments. — The  thick  band,  just  spoken  of,  is  not  a 
separate  structure,  but  only  a  greatly-thickened  strip  of  the  anterior 
part  of  the  fascial  cone  which  we  described  first  of  all  and  which 
has  a  continuous  attachment  all  around  the  circumference  of  the 
orbital  outlet.  Its  principal  thickened  bands  are,  to  use  Tenon's 
words,  "singularly  supple  and  elastic,"  and  are  called  the  internal 
and  external  "check  ligaments." 

Sappey  has  described  smooth  muscular  fibers  in  them  close  to 
their  orbital  attachment. 


The  Globe  and  Its  Socket  21 

Anteriorily  they  are,  through  the  periosteum,  rigidly  fixed  to 
the  orbit  :  by  their  posterior  extremities  they  are  attached  to — 

(a)  The  outer  layer  of  the  sheath  of  the  muscle,  which,  it 
will  be  remembered,  is  part  of  the  fascial  cone  described  first 
of  all  ; 

(6)  The  muscle  itself  through  both  fibrous  and  muscular 
attachments  to  the  belly  of  the  muscle  in  that  region  ; 

(c)  To  the  posterior  hemisphere  of  the  fascial  investment  of 
the  globe  (P.  E.  C. )  by  means  of  a  crescentic  thickening  of  the 


Int.  C.L. 


Int.  R.-*^   ^aaBOfao^     — • 

'  .Ext.  R. 


D, 


Fig.  4 


Tenon's  Fascia  (from  Motais).  Int.  C.  L.  and  Eit.  C.  L. — Internal  and  External  Check  Liga- 
ments. Int.  R:  and  Eil.  R.— Internal  and  External  Kecti.  A.  E.  C.  and  /'.  E.  C. — 
Anterior  and  Posterior  External  Capsule.  /.  C.  L.-  Ifltra-capoular  Ligament,  or 
"Collarette."  /.  C. — Internal  Capsule.  D. — Deep  Layer  of  Muscular  Sheath. 

deep  layer  of  the  muscular  sheath  just  where  it  is  reflected  (/  C.  L.  ) 
back  on  the  globe.  The  check  ligament  draws  on  the  horns  of 
this  fibrous  crescent  (called  by  Lockwood  the  "  intra-capsular 
ligament"  and  by  Motais  the  "  collarette "),  past  the  edges  ot 
the  muscle,  so  that  some  suppose  it  to  act  as  a  kind  of  pulley  or 
stirrup  over  which  the  muscle  works  and  which  keeps  it  from  exert- 
ing injurious  pressure  on  the  globe  during  its  contraction. 

Uses  Of  the  Check  Ligaments.— Besides  this  action,  it  is  evi- 
dent that  the  check  ligaments,  by  acting  on  the  posterior  hemi- 
sphere of  Tenon's  capsule,  help  to  draw  the  eye  forward  (like, 
e.  g.,  the  strings  of  a  night  cap)  against  the  backward  traction  of 
the  recti,  and  in  this  they  are  aided  by  the  entire  anterior  portion 
of  the  fascial  cone,  of  which  they  are  only  thickenings. 


22  Tests  and  Studies  of  the  Ocular  Muscles 

By  their  direct  attachments  to  the  recti,  too,  they  moderate  the 
power  of  these  muscles  on  the  eye  even  with  respect  to  the  muscular 
tone  in  the  absence  of  voluntary  contraction.  When  a  muscle  con- 
tracts they  act  very  beautifully,  extending  in  length  and  acting,  no 
doubt,  according  to  Hooke's  law, ' '  ut  tensio,  sic  vis,"  so  as  to  oppose 
greater  and  greater  resistance  to  the  further  contraction  of  the 
muscle,  like  an  elastic  "brake."  (Compare  Figs.  5,  6  and  7.) 

Merkel*  pointed  out  that  when  a  check  ligament  is  divided,  an 
excessive  rotation  of  the  eye  is  permitted,  and  to  this  Motais f  has 
added  proof  that  at  every  stage  of  rotation  less  muscular  power  is 
required  to  produce  the  same  effect  on  the  eye  after  division  of  the 

ligament  than  when  it  is  intact,  so  that 
it  is  not  only  a  check  ligament  but  also 
a  "moderating  agent  of  the  move- 
ments of  the  globe  during  the  whole 
duration  of  muscular  contraction." 

My  thought  is  that  they  help 
to  slow  off  the  movements  of  the 
eye  towards  their  limits,  so  as  to  avoid 
shock  to  its  contents  by  sudden  arrest 
or  by  change  of  direction  of  motion. 
For  when  a  continuous  force  acts  on 
a  moving  body  (unless  the  resistance 

(Motais)  The  Check  Ligaments  during        •  .    ,     N    ., 

the  primary  position  of  the  eye.          increases  proportionately)  there  is  a 

constant  acceleration  of  speed.    Were 

the  muscles  to  act  in  an  unrestrained  way  on  the  eye,  its  motion 
would  be  far  more  rapid  at  the  end  than  at  the  beginning  of  each 
movement.  Owing  to  the  provision  made  against  this,  I  think 
that  the  motion  rather  slows  off  as  the  limits  of  mobility  are 
reached,  a  greater  and  greater  resistance  being  interposed. 

Indeed,  we  may  think  of  the  internal  and  external  recti  as 
each  possessing  two  tendons  of  insertion — one  inextensible  tendon 
attached  to  the  globe,  and  another  highly  extensible  tendon  attached 
to  the  orbit,  like  the  two  limbs  of  a  letter  Y.  When  the  stem  of 
the  Y,  which  represents  the  belly  of  the  muscle,  contracts,  let  us 
see  what  happens.  At  the  commencement  of  a  muscular  contrac- 
tion almost  all  the  force  acts  on  the  globe,  through  the  inextensible 
limb  of  the  Y,  namely,  the  tendon,  since  the  extensible  one  (the 

*Graefe  und  Saemisch,  Band  I.,  p.  59. 

t  Motais  :  "  Anotomie  de  1'appareil  nioteur  de  1'ceil." 


The  Globe  and  Its  Socket  23 

check  ligament)  offers  but  little  resistance,  and  thus  the  early  part 
of  the  movement  of  the  eye  takes  place  with  that  velocity  which  is 
so  valuable  for  the  requirements  of  life  ;  but  as  the  motion  con- 
tinues, more  and  more  of  the  force  is  transferred  from  one  limb  of 
the  Y  to  the  other,  till  at  last  it  nearly  all  acts  on  the  rigid  bone  of 
the  orbit.  The  eye  is  thus  preserved  from  the  development  of 
excessive  kinetic  energy,  which,  it  will  be  remembered,  varies  not 
as  the  speed  merely,  but  as  the  square  of  the  speed.  It  is  also 
preserved  from  excessive  traction  on  its  coats,  which  might  distort  it ; 
and  the  limitation  of  its  arc  of  mobility  is  determined  rot  so  much  by 


Fig.  6 

(Mot  a  is)  The  Check  Ligaments  dur- 
ing partial  contraction  of  the  Ext. 
Rectus  muscle,  the  Int.  Check  Lig- 
ament being  in  a  state  of  maxi- 
mum relaxation,  and  the  Ext.  one 
being  somewhat  stretched. 


Fig.  7 

(Motais.)  To  show  how,  during  full 
contraction  of  the  Ext.  Rectus, 
the  Ext.  Check  Ligament  is 
stretched  to  its  maximum  length, 
and  the  Int.  is  slightly  stretched 
also. 


impediments  acting  against  the  eyeball  itself  as  by  the  restraint 
imposed  on  the  acting  force.  This  is  a  perfect  arrangement. 

The  maximum  extensibility  of  a  check  ligament  is  10  to  12  mm. 
(Motais),  and  this  exactly  agrees  with  the  known  shortening  of  a 
muscle  which  would  require  to  produce  a  maximum  excursion  of 
the  eye,  say,  of  45°  or  50°. 

The  excursion  does  not  cease  because  the  eyeball  itself  is  not 
capable  of  a  more  extreme  rotation,  nor  yet  because  the  muscle 
has  attained  its  maximum  contraction. 

Of  these  two  statements,  the  first  is  proved  by  the  fact  that 
division  of  the  check  ligament  allows  a  super-physiological  effect 
on  the  eyeball*  (Merkel),  and  the  second  by  the  fact  that  the  con- 
traction of  the  rectus  required  for  a  maximum  physiological  excur- 
sion of  the  eye  is  scarcely  more  than  a  quarter  of  its  length,  while 


*  Graefe  und  Saeruisch,  Band  I.,  1874. 


24  Tests  and  Studies  of  the  Ocular  Muscles 

it  is  known  that  striped  muscles  are  generally  in  their  maximum 
contraction  shortened  by  half  their  length.  Motais  pointed  this  out. 

Internal  Capsule  Of  the  Globe. — This  thin,  transparent  mem- 
brane, which  is  represented  in  Figs.  8  and  12,  is  lined  on  its  sclerotic 
side  by  endothelial  cells,  and  envelopes  the  whole  eye  as  far  forward 
as  the  insertions  of  the  tendons. 

By  its  outer  surface,  it  is  attached  to  the  more  resistant  outer 
capsule.  Underneath  the  tendons  of  the  muscles,  however,  it  runs 


A.E.C. 


LL .(*,  .L-* .....:. — •_, '..-^i.-L 
•  j&SS 


Fig.  8 


A  horizontal  section  of  the  Globe  and  its  Membranes  (after Molais).  E.  C.  L.  and  /.  C.  L.— 
External  aud  Internal  Check  Ligaments.  A.  E.  C.  and  P.  E.  C. — Anterior  and  Posterior 
portions  of  the  External  Capsule.  /.,  /'.,  /.  C.,  /.  C'.— Reflection  of  Internal  Capsule  on 
the  tendon.  ti.  B.—  Serous  Bursa. 

forwards  to  the  insertion  of  the  tendon,  and  is  then  reflected  back- 
wards on  to  the  under  surface  (/  C. )  of  the  tendon  and  muscle,  as  far 
as  the  collarette,  and  mounting  round  its  edges  to  gain  its  upper  sur- 
face, it  there  forms  a  serous  bursa  (S.  B.  )  which  is  elongated  antero- 
posteriorly,  and  partitioned  inside  by  filaments  of  cellular  tissue. 

Between  the  inner  capsule  and  the  sclerotic  is  Tenon's  space, 
which  is  really  a  finely-divided  multilocular  lymph  sac.  It  is  in 
principle  the  subarachnoid  space  of  the  eye,  since  the  inner  capsule 
represents  the  arachnoid  mater,  and  the  outer  capsule  the  dura 
mater,  of  the  globe.  These  peri-ocular  membranes  are  indeed 
continuous  with  the  arachnoid  and  dural  envelopes  of  the  brain, 
though  the  continuity  cannot  always  be  demonstrated.  There  are 


25 

thus  two  sheaths  on  the  optic  nerve,  of  which  the  inner  is  con- 
tinuous anteriorly  with  the  internal  capsule  of  the  globe  and  poster- 
iorly with  the  arachnoid  of  the  brain,  and  this  sheath  can  be  distended 
by  injection  from  the  sub-arachnoid  space  ;  the  outer  sheath  of  the 
optic  nerve  is  continuous  anteriorly  with  Tenon's  outer  capsule  of 
the  globe,  and  posteriorly  with  the  dura  mater  of  the  brain. 

Injections  practiced  into  Tenon's  space,  by  Schwalbe  and 
others,  show  that  it  communicates  with  the  supra-chorioidal  space, 
through  the  openings  of  the  sclerotic  which  transmit  the  vasa  vor- 
ticosa.*  In  the  operation  for  strabismus,  it  is  customary  to  pick  up 
Tenon's  capsule  in  the  forceps,  together  with  the  conjunctiva,  and  as 
soon  as  the  scissors  have  entered  within  the  capsule  their  points  move 
freely  within  the  space,  for  the  fine  filamentary  and  areolar  tissue 
which  partially  occupies  it  ("  tunica  adventitia  oculi  ")  offers  almost 
no  resistance  to  their  movement.  Hemorrhage  may  take  place  into 
this  space  from  wounding  one  of  the  muscular  arteries  and  may 
sometimes  envelope  and  protrude  the  whole  of  the  posterior  two- 
thirds  of  the  globe. 

In  certain  conditions  of  health  the  presence  of  liquid  in  Tenon's 
space  can  be  demonstrated  by  pressing  the  eyelid  against  the  globe  from 
the  equator  forwards.  Tenon' s  space  contains  scarcely  any  free  lymph, 
or  its  pressure  would  be  noticed  during  the  operation  for  strabismus. 

The  numerous  delicate  fibrous  connections  which  exist  between 
the  inner  capsule  and  the  sclerotic,  remind  one,  though  much  finer, 
of  the  loose  cellular  tissue  between  the  occipito-frontalis  tendon  and 
the  peri-cranium,  and  no  doubt  play  a  part  in  the  motions  of  the 
eyes,  which  is  very  similar  to  that  which  the  sub-tendinous  tissue 
plays  during  contraction  of  the  occipito-frontalis. 

Movements  of  Tenon's  Capsule. — From  what  we  have  now 
studied  of  Tenon's  capsule,  we  may  see  at  once  that  it  differs 
entirely  from  such  a  bony  socket  as  that  of  the  acetabulutn,  since 
being  fixed  to  the  globe  near  the  cornea,  and  loosely  so  at  the 
optic  nerve,  it  accompanies  the  motions  of  the  globe  to  a  large 
extent.  Not  entirely,  however,  except  just  in  front,  for  its  elasticity 
allows  it  to  "  give  "  in  some  places  more  than  in  others. 

Motais  has  shown  by  careful  experiments  that  the  fatty  tissue 
which  immediately  surrounds  the  globe,  also  to  a  large  extent 
accompanies  its  movements,  and  every  succeeding  layer  moves  less 

*  One  observer  has  denied  that  Tenon's  space  is  a  lymph  space,  on  the  ground  that  he 
could  not  find  an  open  space  at  all  ;  but  this  is  no  proof,  since  the  same  might  be  said  of  the 
lymph  spaces  in  ordinary  areolar  tissue. 


26 


Tests  and  Studies  of  the  Ocular  Muscles 


than  the  one  within  it.  He  suggests  that  the  real  socket  is  formed  by 
the  inside  of  the  eyelids,  since  they  move  least  in  accordance  with 
its  motions  ;  but  the  fact  of  the  matter  is  that  the  eye  is  an  organ 
sui generis,  and  must  not  too  closely  be  compared  to  a  bony  joint. 
When  we  remember  the  elastic  nature  of  its  connections,  it  strikes 
us  as  exceedingly  well  poised  that  its  center  of  motion  should 
remain  so  stationary  while  acted  on  by  such  various  muscles. 

Why  the  External  Check  Ligament  Should  be  Thicker  and  More 
Powerful  than  any  other,  is  not  at  first  sight  evident,  but  it  may 
possibly  be  explained  by  the  fact  that  the  ocular  muscles  all  rise 
nearer  the  median  plane  than  they  are  inserted,  and  unless  some 
special  provision  existed,  the  eyeball  as  a  whole  might  be  drawn 
too  much  inwards.  As  it  is,  however,  it  is  poised  in  the  aponeurotic 
funnel,  or  fascial  cone  which  extends  from  the  margin  to  the  apex 
of  the  orbit,  and  the  outer  part  of  this  funnel  between  the  globe  and 
the  orbit  is  endowed  with  greater  strength  than  any  other  portion. 
Bearing  of  the  Check  Ligaments  on  Tenotomy. — Motais  called 
attention  to  the  way  in  which  the  check  ligaments  affect  the  result 
obtained  by  a  tenotomy,  and  the  following  are  almost  his  own  words  : 
"  The  tendon,  say,  of  the  internal  rectus,  is  cut.  Immediately, 

by  its  tonicity  alone,  the  muscle 
retires  backwards,  drawing  the 
tendon  with  it,  it  may  be,  let  us 
say,  5  mm. 

"The  check  ligament  adher- 
ing on  one  hand  to  the  muscle, 
and  fastened  on  the  other  hand 
to  the  orbit,  can  only  lend  itself 
to  the  retreat  of  the  muscle  by 
elongating  5  millimeters  (compare 

Fig  9)- 

"Henceforth,  therefore,  in 
consequence  of  the  new  anatomical 
conditions  introduced  by  the  tenot- 
omy, the  check  ligament,  during 
muscular  repose, \v\\\  already  be  ex- 
periencing an  elongation  of  5  mm. 
But  we  know  that  its  maximum  elongation  is  not  greater  than 
10  to  12  mm.  It  has,  consequently,  only  5  to  7  mm.  of  further 
lengthening  at  its  disposal,  during  muscular  contraction.  There 


Fig.  9 

(Molais.)  To  show  the  effect  of  Tenot- 
omv,  M  being  the  Muscle  before, 
and  M'  after,  the  operation.  O-Lis 
the  Check  Ligament  before  opera- 
tion, and  C  L'  the  same  elongated 
after  operation  by  recession  of  the 
muscle. 


The  Globe  and  Its  Socket 


27 


results  a  proportionate    insufficiency    of    adduction,   a   diminution 
in  the  arc  of  rotation. 

' '  But  that  is  not  all.  The  tension  of  the  ligament,  feeble  at  the 
commencement  of  elongation,  gradually  increases.  The  more  it  elon- 
gates, the  greater  becomes  its  tension,  the  more  energetic  becomes 
its  resistance  to  muscular  action. 

' '  If  the  tenotomy  has  already 
produced  a  lengthening  of  the  liga- 
ment 5  mm.,  the  muscle  from  the 
beginning  of  its  contraction  will  be 
restrained  by  a  ligament  already 
considerably  stretched.  Its  con- 
tractile power  will,  by  just  so 
much,  be  lessened. 

' '  Therefore,  we  shall  have  at 
once  from  the  ligament,  diminu- 
tion of  the  extent  and  of  the 
energy  of  the  muscular  action. 
In  advancements,  the  same  phe- 
nomena occur  in  the  opposite 
sense. 

"The   ligament   is  advanced 
at  the  same  time  as  the  muscle.    In 
its  new  position  its  two  points  of  orbital  and  muscular  insertion  be- 
ing brought  nearer  to  each  other,  it  is  of  course  relaxed  (Fig.  10). 

"If  the  advancement  be  3  mm.  the  ligament  will  not  reach  its 
maximum  extension  till  3  mm.  later,  if  I  may  so  express  it. 

"Further,  the  ligament,  being  completely  relaxed  at  the 
beginning  of  contraction,  during  the  first  three  millimeters  will  be 
slack  to  resist  the  muscular  contraction.  We  shall  have,  therefore, 
at  the  same  time,  an  increase  of  the  extent  and  of  the  energy  of 
the  muscular  action."* 

All  the  muscles  of  the  globe  seem  to  be  provided  with  some- 
thing answering  to  check  ligaments.  The  following  account  of 
them  is  taken  chiefly  from  Motais  : 

External  Check  Ligament  (Aileron  ligamenteux  externe).— 
This  is  shown  as  Ext.  C.  L.  in  Fig.  4,  and  E.  C.  L.  in  Fig.  8, 
and  is  a  thick,  grayish-wrhite  band,  which  leaves  the  external  rectus 

*"  L'appareil  moteur  de  1'oeil  "  (1887),  pp.  147  to  150. 


Kig.  1O 

(Motau. )  To  show  the  effect  of  Ad- 
vancement, M  being  the  Muscle 
before,  and  M '  after,  the  operation. 
CL  is  the  Check  Ligament  before 
the  operation,  and  C  L'  the  same 
relaxed  after  operation. 


28  Tests  and  Studies  of  the  Ocular  Muscles 

muscle  near  its  anterior  extremity,  proceeding  forwards  and  slightly 
outwards,  continuing  in  very  nearly  the  same  direction  as  the 
belly  of  the  muscle,  to  the  outer  margin  of  the  orbit. 

Its  mean  breadth  is  7  or  8  millimeters  ;  its  length  from  the 
farthest  back  point  of  its  adherence  to  the  muscle  is  from  1 8  to  20 
millimeters.  Its  greatest  thickness,  which  varies  between  3  and  6 
millimeters,  is  at  its  orbital  insertion.  These  are  the  figures  given 
by  Motais.  He  adds  that  it  is  not  formed  of  a  compact  bundle, 
but  of  a  great  number  of  compact  fascicles,  some  of  which  are  very 
thin.  In  its  posterior  two-thirds  it  is  composed  of  a  mixture  of 
fibrous  and  elastic  tissue  :  in  its  anterior  third  M.  Sappy  discovered 
numerous  smooth  muscular  fibres. 

The  sheath  of  the  external  rectus  muscle,  thin  and  cellular  at 
the  back  of  the  orbit,  becomes  more  and  more  compact  as  we 
trace  it  forwards  along  its  belly.  In  its  posterior  two-thirds  it  is 
loosely  attached  to  the  muscle.  But,  all  of  a  sudden,  about  20 
millimeters  from  the  sclerotic  insertion  of  the  muscle,  it  thickens 
considerably  and  plants  itself  on  the  muscle  so  firmly  that  in 
detaching  it  we  always  tear  some  of  the  muscular  fibres.  These 
adhesions  extend  forwards  5  or  6  millimeters.  The  muscle  then 
changes  its  direction  to  incline  inwards  towards  its  sclerotic  inser- 
tion. The  check  ligament,  instead  of  following  the  curve  of  the 
muscle,  abandons  it,  at  an  angle  which  varies  according  to  the 
position  of  the  globe,  to  reach  the  margin  of  the  orbit,  where  its 
insertion  has  a  breadth  of  7  or  8  millimeters,  and  a  depth  of  3  to 
6  millimeters.  The  upper  border  of  the  ligament  is  reinforced  by 
a  band  from  the  superior  check  ligament.* 

Internal  Check  Ligament  (Aileron  ligamenteux  interne).— 
This  is  shown  as  Int.  C.  L.  in  Fig.  4,  and  /  C.  L.  in  Fig.  8,  and 
is  broader,  but  thinner,  than  the  external  check  ligament.  It 
has  no  interstices  like  the  latter.  Its  color  is  a  yellowish  gray,  and 
near  the  orbital  margin  a  pale  red. 

Though  the  prominence  which  forms  it  is  much  less  differen- 
tiated from  the  neighboring  parts  of  the  aponeurosis  than  that  of 
the  external  ligament,  it  can  easily  be  distinguished  when  it  is  put 
on  the  stretch  by  drawing  on  the  internal  rectus  muscle  behind. 

Its  breadth  is  from  8  to  10  millimeters. ,  Its  length  from  the  pos- 
terior extremity  of  its  attachment  to  the  muscle  to  its  bony  insertion 

*  Panas  says  that  between  this  check  lipament  and  the  corresponding  palpebral  ligament, 
and  the  suh-conjuiictival  fascia,  there  exists  a  space  containing  fat,  and  the  small  accessory 
lachrymal  gland,  which,  he  says,  we  constantly  discover  ill  the  operation  of  cauthopla^ty. 


The  Globe  and  Its  Socket 


29 


"  along  the  line  of  the  crista  lacrimalis  posterior  and  the  wall  of  the 
orbit  just  posterior  to  this  line"  (Howe),  is  from  15  to  18  millime- 
ters. Its  thickness  is  from  i  to  1^2  millimeters,  near  to  its  bony  inser- 
tion. In  very  fatty  orbits,  if  the  muscles  are  atrophied,  it  is  the  least 
distinct  of  all  the  check  ligaments.  Panas  says  that  it  is  fused  (by  an 
expansion  which  covers  Horner's  muscle)  with  the  internal  palpebral 

L.P. 


LSt#.  0. 


R. 


Ext.  R 


S.R. 


Inf.  R. 


(After  Muiii MM 


ligament,  so  that  when  the  right  internal  rectus  contracts,  it  draws 
back  the  inner  commissure  of  the  lids,  the  semilunar  membrane  and 
the  caruncle,  at  the  same  time  that  it  compresses  the  lachrymal  sac. 
Superior  Check  Ligaments. — These  are  two.  Owing  to  the 
broad  tendon  of  the  levator  palpebrae  being  interposed  between  the 
superior  rectus  and  the  orbital  margin,  the  superior  check  ligaments 
from  the  superior  rectus  cannot  reach  their  orbital  insertions  except 
by  passing  each  border  of  the  levator  palpebrse  tendon.  Were 
there  a  single  median  check  ligament,  it  would  have  to  pierce  this 
intervening  tendon  to  reach  the  bone  (see  Fig.  n). 


30  Tests  and  Studies  of  the  Ocular  Muscles 

That  this  affords  an  explanation  for  there  being  two  superioj 
check  ligaments  is  shown  by  the  fact,  noted  by  Motais,  that  those 
vertebrata  which  possess  a  levator  palpebrae  have  these  ligaments 
double,  whereas  those  which  possess  no  levator  have  a  single 
median  ligament. 


1.0.  l.L. 


Fig.  13 


I.E. 


(After  Motate).  Vertical  longitudinal  section  of  Eyeball  and  adnexa.  R.— Reflexion  of  fascia 
from  the  under  surface  of  the  Levator  Palpebne  on  to  the  upper  surface  of  the  Superior 
Kectus.  C.-  Part  of  the  funnel-shaped  expansion  proceeding  to  the  margin  of  the  orbit. 
/".—Expansion  to  contribute  to  the  anterior  part  of  the  capsule.  /.  R.—  Inferior  Kectus. 
1.  O.—  Inferior  Oblique.  /.  L.— Check  Ligament  of  the  Inferior  Rectus,  embracing  the 
Inferior  Oblique. 

He  describes  the  internal  superior  check  ligament  (/  S.)  as  a 
fibrous  cord  which  leaves  the  inner  border  of  the  superior  rectus 
muscle,  applies  itself  to  the  tendon  sheath  of  the  superior  oblique 
muscle,  and  is  inserted  with  it  at  the  trochlea.  Sometimes  a  few 
muscular  fibres  run  into  it,  and  in  any  case  it  is  intimately  adherent 
to  the  muscle,  just  as  the  internal  and  external  check  ligaments  are 
to  theirs. 

The  external  superior  check  ligament  (E.  S.~)  is  a  more 
flattened  band  than  the  preceding  cord,  and  divides  into  two,  one 


L.I.R.. 


portion  joining  the  upper  border  of  the  external  check  ligament 
and  the  other  portion  reaching  the  orbital  margin  midway  between 
this  and  the  outer  extremity  of  the  tendon  of  the  levator  palpebrae. 

The  Inferior  Check  Ligaments  are  also  two,  but  are  a  little 
difficult  to  understand. 

That  of  the  inferior  rectus  leaves  the  sheath  and  belly  of  the 
inferior  rectus  at  the  point  where  that  muscle  begins  to  curve  round 
the  globe,  adhering  intimately  to  the  muscle  and  to  its  thickened 
shealh,  for  a  length  of  5  or  6  millimeters  (see  /  L.  in  Fig.  12). 

It  proceeds  to  the  middle  part  of  the  inferior  oblique  muscle 
(/  O.  ),  splitting  into  two  so  as  to  embrace  it,  as  shown  in  the  figure, 
establishing  thus  a  strong  connection  between  these  two  muscles. 

Its  appearance 
is  whiter  and  its 
structure  more  ex- 
clusively fibrous 
than  that  of  the 
other  check  liga- 
ments, so  that,  with 
a  moderate  thick- 
ness, it  is  as  resis- 
tant as  the  external 
check  ligament 
itself  (Motais).  It 
obtains  no  direct  Int. 
insertion  into  the 
margin  of  the  orbit, 
but  only  through  a 

loop,    of    which    one  (Motais.)    View  of  the  under  surface  of  the  Eyeball,  the  floor  of 

,       .  the  orbit  being  removed  to  show  the  Check  Ligament  (L.O. ) 

limb    IS     formed     by  of   the   Inferior  Oblique    muscle  (/.  O.).      These    form   two 

.           .  limbsof  a  Y,  the  stem  of  which  isthe  Check  Ligament  (L.I.R.) 

the    inferior    Oblique  of  the  Inferior  Rectus  {Inf.  R.),  seen  to  embrace  the  Inferior 

,                             ?  Oblique.     Int.  R. — Internal  Rectus.     S.  R. — Superior  Rectus. 

muscle  and  the  other 

limb  by  the  check  ligament  of  that  same  muscle.  This  is  shown 
in  Fig.  13.  For  this  reason  it.  is  that  Fig.  12  looks  as  if  the  check 
ligament  of  the  inferior  rectus  had  no  insertion  except  its  slender 
offshoot  to  the  eyelid,  which,  however,  accounts  for  the  eyelid 
being  drawn  down  during  contraction  of  the  inferior  rectus. 

We  have  seen  that  it  embraces  the  inferior  oblique  muscle,  in 
Figs.  12  and  13.  From  the  point  where  it  does  so,  springs  the 
structure  we  have  next  to  consider. 


32  Tests  and  Studies  of  the  Ocular  Muscles 

The  Check  Ligament  of  the  Inferior  Oblique. — This  fibrous 
bundle  (L.  O.  in  Fig.  n),  derived  in  part  from  the  fibres  of  the 
check  ligament  of  the  inferior  rectus,  in  part  from  the  sheath  of  the 
inferior  oblique  muscle,  leaves  the  anterior  border  of  the  inferior 
oblique  about  8  or  10  millimeters  from  its  orbital  origin,  and  from 
thence  courses  obliquely  outwards  and  forwards.  It  forms  an 
obtuse  angle  of  about  120°  with  the  check  ligament  of  the  inferior 
rectus  muscle.  With  the  inferior  oblique  it  forms  an  angle  of 
about  110°.  Its  length  is  from  10  to  12  millimeters,  and  it  is 
inserted  into  the  lower  outer  angle  of  the  orbit,  4  or  5  millimeters 
behind  the  orbital  margin,-  about  midway  between  the  external 
check  ligament  and  the  origin  of  the  inferior  oblique. 

This  bundle  is  the  most  pearly  looking,  the  most  purely  fibrous 
of  all  the  aponeurotic  lamellae  of  the  orbit. 

Its  breadth  varies  at  different  parts  of  its  course  :  in  the  middle, 
2  or  3  millimeters  ;  at  its  muscular  insertion,  7  or  8  millimeters  ;  at 
its  bony  insertion,  5  or  6  millimeters.  It  presents,  therefore,  the 
shape  of  two  triangles  united  by  their  apices. 

Motais  thinks  it  acts  not  only  as  a  moderator  for  the  inferior 
oblique,  but  also  as  a  "pulley  of  reflection."*  Together  with  the 
inferior  oblique  itself,  it  forms  a  kind  of  musculo-aponeurotic  loop, 
the  two  ends  of  which  are  inserted  near  the  orbital  margin,  one  at 
the  outer  angle,  the  other  at  the  inner  angle.  And  the  check  liga- 
ment of  the  inferior  rectus  muscle  embraces  the  middle  portion  of 
this  loop,  so  that  when  the  inferior  rectus  begins  to  contract,  its 
check  ligament  stresses  the  loop. 

The  check  ligament  of  the  inferior  rectus  has  therefore  for  its 
orbital  insertions  the  tendon  of  the  inferior  oblique  muscle,  and  the 
check  ligament  of  the  same  muscle  like  the  two  limbs  of  a  Y. 

The  Connection  of  the  Levator  Palpebrae  Muscle  with  the 
Superior  RectUS  deserves  a  passing  notice,  since  these  muscles 
work  so  uniformly  together. 

From  the  upper  surface  of  the  sheath  of  the  superior  rectus, 
near  its  inner  border,  and  along  its  whole  length  from  the  apex  of 
the  orbit  to  the  equator  of  the  eyeball,  is  given  off  a  sheet  of 
fibro-cellular  tissue,  which  reaches  the  under  surface  of  the 
levator.  and  splits  into  two  to  enclose  that  muscle,  thus  providing 
it  with  a  sheath. 

*  By  this  he  must  mean  that  when  the  muscle  contracts,  the  ligament  slightly  bends  the 
muscle  by  drawing  its  middle  part  outwards,  so  as  to  make  its  traction  on  the  eye  a  little  less 
oblique. 


The  Globe  and  Its  Socket  33 

On  reaching  the  equator,  however,  the  upper  surface  of  the 
sheath  of  the  superior  rectus  is  reflected  {R  in  Fig.  12)  as  a  whole 
on  to  the  under  surface  of  the  levator,  describing  a  strong  curve, 
with  concavity  backwards,  in  its  passage  from  one  muscle  to  the 
other.  A  prolongation  (P),  however,  still  continues  to  cover  the 
outer  or  upper  surface  of  the  tendon  of  the  superior  rectus,  and 
forms,  in  fact,  part  of  the  anterior  hemisphere  of  the  external 
capsule  of  the  eye  continuous  with  A  E  C  in  Fig.  4.  From  the 
upper  surface  of  the  levator  is  given  off  a  facial  layer  (C),  which 
goes  to  the  orbital  margin,  and  forms  part  of  that  facial  cone  in  the 
orbit  which  we  commenced  this  whole  subject  by  describing. 
Notice,  too,  that  the  nerve  for  the  levator  penetrates  the  superior 
rectus. 


CHAPTER    II 


The  Ocular  Motions 

A  universally  mobile  body  is  capable  of  no  fewer  than  six  inde- 
pendent motions,  which  are  called  "  degrees  of  freedom."  It  can  be 
translated  as  a  whole  in  any  three  directions  at  right  angles  to  each 
other,  or  be  rotated  about  any  three  axes  at  right  angles  to  each  other. 

Translations  Of  the  Globe. — If  we  regard  the  head  as  fixed,  and 
confine  ourselves  to  the  study  of  the  voluntary  motions  of  the  eye- 
ball, we  shall  find  it  approximately  true  that  translation  of  the  globe 
is  forbidden  in  virtue  of  its  attachments  to  the  orbit. 

Were  we  to  investigate  this  statement  very  strictly,  we  should 
not,  however,  find  it  rigidly  true,  since  the  center  of  motion  lies  a 
little  farther  back  than  the  geometrical  center  of  the  eyeball,  in  con- 
sequence of  which  the  globe  is  slightly  translated  in  whatever  direc- 
tion the  eye  is  made  to  turn.  On  looking  to  the  right,  the  globe  is 
translated  slightly  to  the  right  ;  on  looking  to  the  left,  to  the  left, 
and  so  on.  In  the  maximum  excursions  of  the  eye,  this  translation 
is  probably  not  less  than  I,  or  greater  than  2  millimeters. 

Center  Of  Motion. — The  distance  between  the  mid-point  of 
the  optic  axis  and  the  center  of  rotation  is  given  by  Bonders  and 
Mauthner  as  follows  : 


R  EFR  ACTION 

DONDERS 

MAUTHNER 

In  Krnmetropia    

i  77  mm. 

1.24  mm. 

"  Myopia  

1.75  mm. 

1.82  mm. 

"   Hypermetropia  

2.17  mm. 

1.47  mm. 

Since,  except  to  the  trifling  extent  just  noticed,  translation 
is  denied  to  the  eye,  we  may  now  turn  our  attention  to  its 
rotations. 

Rotations. — A  body  deprived  of  translation  might  still  be  able 
to  rotate,  and  that  about  three  axes  at  right  angles  to  each  other. 
Rotations  about  all  other  axes  are  resolvable  into  rotations  about 


The  Ocular  Motions  35 

two  or  more  of  these,  from  which  it  follows  that  a  body  which 
enjoys  three  degrees  of  rotational  freedom  can  rotate  about  as 
many  diameters  as  are  conceivable. 

We  have,  therefore,  next  to  inquire  whether  the  eyeball 
retains  this  full  rotational  freedom. 

One  Voluntary  Rotation  Denied. — Actual  experiment  has  shown, 
what  we  could  not  have  otherwise  proved,  that  one  degree  of  free- 
dom is  lost  in  all  voluntary  parallel  movements  of  the  eyes  which 
start  from  the  straight  forward  position.* 

The  degree  of  freedom  lost  is  that  of  rotating  about  the  fore- 
and-aft  axis  (considered  as  fixed  in  the  head),  while  the  two  free- 
doms retained  are  those  of  rotation  about  the  vertical  axis,  and 
about  the  transverse  axis  (both  considered  as  fixed  in  the  head). 

Listing's  Plane. — Simultaneous  rotations  about  the  vertical  and 
transverse  axes  can  be  variously  compounded  into  rotations  about 
any  intermediate  axis.  This  is  equivalent  to  saying  that  they  are 
limited  to  rotations  about  all  conceivable  diameters  in  one  plane, 
namely,  that  plane  in  which  the  vertical  and  transverse  axes  lie,  and 
which  it  is  convenient  to  call  "  Listing's  plane,"  since  this  degree 
of  constraint  was  discovered  by  Listing. 

Listing' s  plane  passes  through  the  center  of  motion  of  the 
eyes,  and  is  a  vertical  transverse  plane  (corresponding  to  a  coronal 
section)  fixed  in  the  head,  and  perpendicular  to  the  fore-and-aft 
axis,  about  which  rotation  is  denied,  f 

When  the  head  is  held  erect  and  the  eyes  look  straight 
forward  at  a  very  distant  object  on  the  horizon,  they  are  gener- 
ally said  to  be  in  their  "primary  position,"  and  though  we  shall 
have  to  quote  a  truer  definition  later  on,  we  may  for  the  present 
accept  this  simple  one,  in  order  to  say  that  however  many  and 
complex  the  motions  of  an  eye  may  be  in  glancing  from  point  to 
point,  the  ultimate  result  of  them  all  is  equivalent  to  a  single  rota- 
tion of  the  globe  about  some  one  axis  in  Listing's  plane,  provided 
the  eye  has  started  from  the  primary  position.  J 

Torsion. — By  torsion  we  mean  rotation  of  the  eyeball  about  its 
own  fixation  line.§ 

*  Latent  Torsion,  discussed  in  Chapter  XIII.,  is  not  voluntary. 

tin  the  "primary  position"  of  the  eye,  Listing's  plane  is  practically  identical  with  the 
"equatorial  plane"  of  the  eye,  but  it  must  not  be  identified  with  it,  since  the  latter  moves 
with  the  eye,  whereas  Listing's  plane  does  not. 

Jit  will  he  seen  that  I  have  guarded  myself  from  stating  that  rotations  from  one 
secondary  position  to  another  are  about  axes  in  Listing's  plane.  They  are  not.  Helmholtz 
has  correctly  shown  in  what  plane  they  lie. 

§  "  We  will  call  torsions  rotations  of  the  eye  about  the  line  of  fixation  "  ( Helmholtz) . 


36  Tests  and  Studies  of  the  Ocnlar  Muscles 

Let  us  remember  that  there  are  two  fore-and-aft  axes  we  have 
to  consider,  one  of  which  is  fixed  in  the  head  and  which  we  have 
already  treated,  and  another  proper  to  the  eyeball  itself  and  moving 
with  it,  so  as,  indeed,  to  be  for  all  practical  purposes  regarded  as 
identical  with  the  fixation  line. 

Secondary  Torsion. — When  the  eyeball  (starting  from  the  pri- 
mary position)  rotates  either  vertically  upwards  or  downwards, 
or  horizontally  to  either  side,  its  motions  are  called  "cardinal 
motions,"  and  are  not  accompanied  by  torsion.  But  when  the 
eyeball  looks  obliquely,  in  any  intermediate  direction,  two  cardinal 
motions  are  compounded  together.  Every  motion  of  an  eye  from 
the  primary  into  an  oblique  position  is  accompanied  by  torsion  as 
an  essential  component  of  the  motion. 

Bonders'  Law. — Bonders'  observed  that  whatever  position  the 
eyeball  may  take,  there  belongs  to  that  position  a  definite  amount 
of  torsion  which  remains  the  same  no  matter  how  often  the  eye 
may  return  to  that  position,  and  however  many  motions  it  may 
make  in  arriving  at  it. 

To  quote  his  own  words  :  "For  any  determinate  position  of 
the  line  of  fixation  with  respect  to  the  head,  thereto  corresponds  a 
determinate  and  invariable  angle  of  torsion,  a  value  independent 
of  the  volition  of  the  observer,  independent  also  of  the  manner  in 
which  the  line  of  fixation  has  been  brought  into  the  considered 
position. ' ' 

The  same  law  has  been  put  more  concisely  by  Helmholtz  (and 
at  the  same  time  amplified)  in  the  words  :  "  The  wheel-movement 
of  each  eye  is,  with  parallel  fixation  lines,  a  function  only  of  the 
elevation  angle,  and  of  the  lateral  deflection  angle."* 

Listing's  Law. — The  law  of  Listing  goes  a  step  further  than 
that  of  Bonders,  and  is  as  follows  :  "  When  the  line  of  fixation 
passes  from  its  primary  to  any  other  position,  the  angle  of  torsion 
of  the  eye  in  /his  second  position  is  the  same  as  if  the  eye  had 
arrived  at  this  position  by  turning  about  a  fixed  axis  perpendicular 
to  the  first  and  second  positions  of  the  line  of  fixation.''1 

This  simply  means  that  when  the  eye  starts  from  the  primary 
position  and  glances  towards  an  object  situated  obliquely  (^.  g. , 
up  and  to  the  right),  the  line  of  fixation  takes  the  shortest  possible 
cut  to  its  new  direction,  and  in  so  doing  must  necessarily  sweep 
along  a  plane  common  to  its  original  and  its  new  position.  To 

"Helmholtz's  "  Optique  Physiologique,"  page  602. 


The  Ocular  ^fotions  37 

permit  it  to  do  this  the  eyeball  must  rotate  around  an  axis  perpen- 
dicular to  this  plane  and,  therefore,  perpendicular  to  the  line  of 
fixation  throughout  the  whole  of  its  motion.  Since  the  shortest 
cut  requires  the  briefest  time  to  traverse,  it  is  manifest  that  this  law 
is  essential  to  the  perfection  of  the  ocular  movements  where  rapidity 
is  so  advantageous.  The  exquisiteness  of  this  design  is  apparent 
when  one  considers  that  no  fewer  than  three  muscles  are  concerned 
in  every  oblique  motion  of  the  globe,  not  one  of  which,  acting 
individually,  would  rotate  the  eye  about  the  required  diameter. 

Reasons  for  Listing's  Law. — I  think  a  little  consideration  will 
show  that  the  arrangement  on  which  Listing's  law  is  based  is  that 
which  entails  the  absolute  minimum  of  motion  (calculated  as  the 
sum  of  the  motions  of  all  the  particles  of  the  eye),  so  that  (#)  the 
momentum  of  the  ocular  contents  is  the  least  possible  ;  (<5)  the  time 
is  the  shortest  ;  (V)  the  work  done  the  least,  and  (^)  the  lowest 
amount  of  dangerous  "kinetic  energy"  is  developed.  A  second, 
more  important,  reason  is,  that  by  no  other  equally  efficient  arrange- 
ment could  the  law  of  Donders  be  possible,  since  the  torsion  belong- 
ing to  each  position  of  the  eye  would  not  be  a  constant  quantity, 
and  this  would  throw  the  brain  out  in  its  calculations. 

Proof  Of  Listing's  Law. — The  truth  of  this  law  has  been  con- 
firmed (within  the  sphere  in  which  it  holds  good,  namely,  that  of 
the  parallel  motions  of  the  eyes)  by  every  observer  that  has  under- 
taken to  test  it  by  actual  observation. 

It  is  desirable  to  study  the  motions  of  the  eyes  before  com- 
mencing to  consider  the  muscles  by  means  of  which  they  are 
brought  about.  This  will  save  us  from  falling  into  errors  from 
failure  to  distinguish  between  motions  actually  observed  and  those 
which  our  preconceived  notions  of  the  oblique  muscles  might  make 
us  think  ought  to  take  place. 

A  most  delicate  means  of  following  the  para/lei  movements  of 
the  eyes,  attributed  to  Rente,  is  afforded  by  the  experimental  use 
of  ' '  after-images. ' '  The  following  mode  of  inquiry  thereby  is 
that  most  to  be  recommended  : 

Let  the  experimenter  affix  vertically  a  scarlet  ribbon,  two  or 
three  feet  long,  to  a  gray  wall,  with  the  center  of  the  ribbon  at  the 
same  height  as  his  own  eye,  when  seated  at  some  distance  there- 
from, and  with  his  head  erect  and  squarely  facing  the  wall  let  him 
gaze  at  the  center  of  the  ribbon  for  a  minute  or  two.  On  now 
raising  his  gaze  directly  upwards,  while  keeping  his  head 


38  Tests  and  Studies  of  the  Ocular  Muscles 

immovable,  a  faint  after-image  will  move  upwards  with  the  eyes,  but 
will  remain  strictly  vertical.  On  lowering  his  gaze,  the  after-image 
will  sink  simultaneously  but  still  remain  vertical.  If,  however,  after 
raising  his  gaze  he  were  to  turn  his  eyes  to  the  right,  the  after- 
image would  no  longer  remain  vertical,  but  would  slope  to  the  right  ; 
on  looking  up  to  the  left,  it  would  slope  to  the  left ;  on  looking 
down  to  the  right  its  upper  end  would  again  slope  to  the  left,  and  on 
looking  down  to  the  left  its  upper  end  would  slope  to  the  right. 

We  might  conclude  from  this  that  when  the  eyes  occupy 
oblique  positions  they  experience  torsion  equal  to  that  of  the  after- 
image. But  since  those  parts  of  the  wall  upon  which  the  image 
falls  in  these  positions  are  not  perpendicular  to  the  line  of  sight,  the 
slope  is  exaggerated  and  the  proof  is  not  complete. 

To  vary  the  experiment,  commence  again,  but  with  the  head 
rotated  considerably  to  the  left  and  kept  immovably  so.  Now,  after 
gazing  at  the  ribbon,  run  the  eyes  up  the  wall  immediately  above 
it,  when  the  image  will  appear  to  become  more  and  more  twisted  to 
the  right  the  higher  it  is  raised.  This  proves  infallibly  that  torsion 
does  take  place  on  looking  up  and  to  the  right,  though  the  amount 
is  less  than  the  previous  experiment  would  have  led  us  to  suppose. 
Similar  experiments  could  be  made  for  the  other  oblique  positions 
of  the  head,  which  show  that  torsion  to  the  right  occurs  on  looking 
upwards  and  to  the  right,  or  downwards  and  to  the  left.  On  the 
contrary,  torsion  to  the  left  occurs  on  looking  upwards  and  to  the 
left,  or  downwards  and  to  the  right. 

Even  this  experiment,  however,  though  it  correctly  indicates 
the  presence  of  torsion  and  the  true  sense  in  which  it  occurs,  does 
not  enable  us  to  measure  it  exactly,  because  the  after-image  is  pro- 
jected on  a  plane  which  is  not  perpendicular  to  the  visual  line  in  the 
secondary  position.  To  rectify  this,  let  a  gray  drawing  board  be 
suspended  near  the  ceiling,  directly  above  the  scarlet  ribbon,  and 
be  provided  (according  to  a  method  of  Le  Conte's)  with  a  large 
knitting  needle  projected  perpendicularly  from  its  center.  Now 
make  the  drawing  board  lean  forward,  like  a  picture,  until  this 
needle  is  seen  "end  on"  by  the  experimenter.  On  looking  at 
the  needle,  the  after-image  is  projected  on  to  the  board  in  a  manner 
which  represents  the  exact  torsion  of  the  eye.  It  can,  if  we  like,  be 
measured  by  a  long  wire  so  attached  by  its  middle  to  the  foot  of  the 
knitting  needle  as  to  rotate  against  the  board  to  any  required  angle. 
If  this  wire  be  inclined  by  an  assistant  till  it  exactly  coincides  with 


The  Ocular  Motions  39 

the  after-image,  its  deflection  from  the  vertical  exactly  measures  the 
torsion  of  the  eye. 

Now  let  him  place  the  ribbon  horizontally  on  the  wall,  and 
holding  the  head  erect  as  at  first,  with  face  square  to  the  wall,  gaze 
at  it  steadily,  and  then  move  the  eyes  horizontally  to  the  right. 
The  image  will  now,  in  most  cases,  appear  slightly  tilted,  with  its 
right-hand  end  dipping.  On  turning  the  eyes  to  the  left,  the  after- 
image will  appear  tilted  in  the  opposite  direction,  the  left-hand  end 
extremely  dipping. 

If  the  face  be  turned  somewhat  downwards,  the  tilting  of  the 
image,  on  looking  to  the  right  or  the  left,  will  be  greatly  increased  ; 
while,  on  the  other  hand,  if  the  face  be  raised  towards  the  ceiling, 
the  sense  of  the  tilt  will  be  reversed  :  on  looking  to  the  right  the 
left-hand  end  dipping  and  on  looking  to  the  left  the  right-hand  end. 
This  fully  confirms  the  results  of  the  previous  experiment  and  shows 
that  whether  we  study  the  vertical  meridian  of  the  retina  or  the 
horizontal  meridian,  torsion  takes  place  in  the  directions  indicated. 

If,  however,  the  head  be  thrown  only  slightly  backwards,  a 
position  is  gained  from  which,  when  the  eyes  look  to  the  right  and 
left,  the  image  no  longer  tilts,  but  remains  strictly  horizontal.  The 
eyes  while  looking  at  the  center  of  the  ribbon  are  now  said  to  be  in 
their  "primary  position,"  which  is  defined  physiologically  as  that 
from  which  motions  of  the  eyes  in  directly  vertical  or  directly 
horizontal  directions  are  unaccompanied  by  torsion.  On  glancing, 
however,  in  oblique  directions  therefrom,  torsion  occurs.* 

After  providing  the  head  of  the  experimenter  with  some  means 
of  fixing  it  with  the  exact  amount  of  backward  tilt,  which  brings 
the  eyes  into  their  primary  position,  the  ribbon  may  be  fixed 
obliquely  on  the  wall,  at  an  angle  of,  say,  45°,  or  any  other.  "What- 
ever the  angle  may  be,  on  turning  the  eyes  in  the  direction  indi- 
cated by  the  length  of  the  ribbon,  the  after-image  will  be  found  to 
remain  in  a  straight  line  with  the  ribbon  in  all  parts  of  its  course. 
This  proves  that  parallel  movements  of  the  eyes  from  their  primary 
position,  in  no  matter  what  direction,  take  place  as  if  about  axes 
at  right  angles  to  each  line  of  fixation  while  in  the  primary  position. 

The  experimenter  may,  if  he  please,  reach  any  point  on  the 
wall  by  a  circuitous  or  even  spiral  route,  but  he  will  always  find,  at 
the  end,  that  the  after-image  occupies  exactly  the  same  position  as 

*  Strictly  speaking,  the  ribbon  should  be  at  an  infinite  distance  ior  the  definition  to  be 
true,  since  only  then  would  the  visual  lines  bu  parallel. 


40  Tests  and  Studies  of  the  Ocular  Muscles 

if   the  eyes  had    been   moved    to    that  position  directly  from    the 
primary  position. 

By  experiments  of  this  kind,  Bonders  arrived  at  his  dis- 
covery of  the  law  that  "to  every  position  of  the  fixation  line  with 
reference  to  the  head  belongs  a  definite  and  unchangeable  value  of 
torsion." 

On  this  law  of  Donders,  everything  else  rests.  The  laws 
of  Listing  and  Helmholtz,  to  be  described  shortly,  are  necessary 
corollories. 

Listing's  law  relates  only  to  those  parallel  movements  of 
the  eyes  which  have  the  primary  position  for  their  point  of 
departure,  and  states  that  the  position  of  the  eye  in  any  secondary 
position  is  what  it  would  gain  by  a  rotation  from  the  primary 
position  about  a  fixed  axis  perpendicular  to  the  primary  and 
secondary  positions  of  the  fixation  line.  All  axes  of  this  kind 
must  lie  in  one  plane,  viz. ,  that  which  is  perpendicular  to  the 
fixation  line  in  its  primary  position,  and  which  has  been  called, 
in  consequence,  Listing's  plane. 

Suffice  to  say  that  any  linear  after-image,  which  possesses, 
during  the  primary  position  of  the  eye,  a  given  obliquity,  when 
projected  on  a  gray  wall  facing  the  observer,  preserves  the  same 
obliquity  inviolate  whenever  the  eye  glances  in  the  direction 
indicated  by  the  length  of  the  false  image,  or  in  a  direction  per- 
pendicular to  its  length.* 

Though  all  observers  are  agreed  as  to  the  truth  of  Listing's 
law,  all  are  not,  however,  agreed  as  to  the  conclusions  to  be  drawn 
from  it. 

Agreement  of  Helmholtz  and  Donders.— A  good  deal  of  dis- 
cussion has  been  made  recently  about  the  discrepancy  which  has 
been  stated  in  America  to  exist  between  the  laws  of  false  torsion 
formulated  by  Helmholtz  and  those  laid  down  by  Donders. 

There  can  be  no  question  that  their  statements,  as  they  read, 
look  diametrically  opposed  to  each  other. 

And  yet  a  careful  study  of  Helmholtz  will  show  that  he  has 
chosen  a  different  definition  and  index  of  torsion,  so  that  his  state- 
ments do  not  really  contradict  those  of  Donders,  but  perfectly 
agree,  as  indeed  we  could  only  have  expected. 

*It  is  true  that  when  the  eye  glances  in  oblique  directions  other  than  these,  the  after- 
image does  appear  to  have  its  degree  of  obliquity  altered,  but  this  is  fully  explained  in  every 
case  by  the  fact  that  it  is  projected  on  a  flat  surface  which  does  not  (in  that  part  of  it)  face  the 
observer,  and  the  apparent  discrepancies,  when  properly  analyzed,  only  confirm  the  law. 


The  Ocular  Motions 


Let  us  look  at  them  in  the  following  parallel  columns  : 


HELMHOLTZ. 

1  When  the  plane  of  fixa- 
tion is  directed  upwards, 
lateral  displacements  to  the 
right  make  the  eye  turn  to 
the  left ; 


BONDERS. 

"  On  the  diagonal  fixation 
upwards  and  to  the  right, 
the  vertical  meridians  of  both 
eyes  suffer  a  parallel  inclina- 
tion to  the  right." 


and  displacements  to  the  left 
make  the  eye  turn  to  the 
right." 


"On  diagonal  fixation 
upwards  and  to  the  left,  the 
vertical  meridians  of  both 
eyes  suffer  a  parallel  incli- 
nation to  the  left." 


"  When  the  plane  of  fix- 
ation is  lowered,  lateral  dis- 
placements to  the  right  are 
accompanied  by  torsion  to 
the  right 


"On  diagonal  fixation 
downwards  and  to  the  right, 
the  vertical  meridians  of  both 
eyes  suffer  a  parallel  inclina- 
tion to  the  left." 


and  vice  versa.'" 


"  On  diagonal  fixation 
downwards  and  to  the  left, 
the  vertical  meridians  of  both 
eyes  suffer  a  parallel  inclina- 
tion to  the  right." 


HelmholtZ's  Plane  Of  Reference.— The  "plane  of  reference" 
adopted  by  Helmholtz  is  the  "visual  plane,"  by  which  he  means 
the  plane  common  to  the  two  visual  axes  and  to  the  line  which 
joins  the  centers  of  motion  of  the  two  eyes.  When  the  visual 
axes  are  elevated  or  depressed,  the  visual  plane  is  elevated  or 
depressed  with  them. 

In  the  primary  position  of  the  eyes,  the  visual  plane  passes 
through  the  horizontal  meridian  of  the  retina,  which  Helmholtz 
calls  the  "retinal  horizon."  In  all  the  cardinal  motions  of  the 
eyes,  which,  it  will  be  remembered,  are  motions  from  the  primary 
position  directly  upwards,  downwards,  to  right  and  to  left,  the 
retinal  horizon  lies  rigorously  in  the  visual  plane  ;  but  in  oblique 
motions  it  becomes  more  and  more  inclined  to  the  visual  plane,  in 
the  sense  stated  by  Helmholtz  in  the  first  of  the  above  parallel 
columns. 

Bonders'  Plane  of  Reference. — I  do  not  know  what  plane  of 
reference  Donders  selected,  but  (since  the  one  which  I  have 
selected  gives  the  same  results)  probably  the  same  as  that  which 


42  Tests  and  Studies  of  the  Ocular  Muscles 

I  have  adopted  in  what  follows,  namely,  a  movable,  ever-vertical 
plane,  passing  through  the  line  of  fixation  and  moving  with  it. 

No  Torsion  with  Reference  to  the  Median  Plane. — Were  we 
to  estimate  torsion  by  reference  to  the  median  plane  of  the  head, 
or  any  plane  parallel  to  it,  we  would  have  to  conclude  there  is  no 
torsion  at  all,  for,  thus  tested,  the  vertical  meridian  of  the  cornea 
would  be  torted  in  one  direction  and  the  horizontal  meridian  to  an 
equal  amount  in  the  opposite  direction. 

Indeed,  it  stands  to  reason  that  motion  about  any  axis  in 
Listing's  plane  cannot  have  a  component  about  a  line  perpen- 
dicular to  that  plane. 

The  nature  of  false  torsion  depends  entirely  upon  the  point  of 
view  from  which  we  observe  it. 

Index  Of  Torsion. — Since  the  eye  is  an  optical  instrument,  I 
think  the  index  of  torsion  should  be  an  optical  one,  and,  to  my 
mind,  the  best  plan  is  to  imagine  the  point  of  fixation,  or,  in  other 
words,  the  object  looked  at,  to  be  an  intelligent  being,  able  to  tell 
us  what  amount  of  torsion  exists  from  his  point  of  view.  The 
torsion  would  thus  be  measured  by  the  angle  between  the  originally 
vertical  meridian  of  the  retina  (i.  e.,  the  meridian  which  was  ver- 
tical in  the  primary  position  of  the  eye)  and  the  vertical  plane  pass- 
ing through  the  line  of  fixation. 

When  calculated  in  this  way,  the  rules  of  false  torsion  agree 
exactly  with  those  of  Bonders,  and  therefore  with  all  the  text-books 
which  have  followed  him. 

Let  us  give  the  name  of  Dextrotorsion  to  that  which  takes 
place  when  the  upper  end  of  the  vertical  diameter  of  the  eyeball 
is  tilted  to  the  patient's  right,  and  Laevotorsion  to  similar  tilting  to 
the  left.  When  we  look  upwards  and  to  the  right  or  downwards 
and  to  the  left,  there  is  dextrotorsion.  Conversely,  when  we 
look  upwards  and  to  the  left  or  downwards  and  to  the  right,  there 
is  laevotorsion.  In  fact,  the  paths  of  the  after-images  trace  out  a 
figure  shaped  like  a  sheaf  of  wheat. 

Torsion  Calculator. — I  have  constructed  a  simple  little  model, 
which,  though  difficult  to  describe  on  paper,  makes  it  very  easy  to 
demonstrate  the  true  nature  of  secondary  torsion  and  even  to  indi- 
cate automatically  its  amount  in  degrees  for  any  oblique  motion 
whatever  of  an  eye. 

In  its  home-made  form  it  consists  of  a  circular  piece  of  card- 
board (shown  in  No.  i  of  Fig.  14),  with  a  vertical  diameter  V  V 


The  Ocular  Motions 


43 


Fig 


The  author's  torsion  calculator.  No.  1. — The  eye  in  its  primary  position.  No.  2. — The  eye 
looking  up  and  to  the  right,  showing  equal  and  opposite  inclinations  of  the  horizontal 
and  vertical  meridians  with  reference  to  the  fore-and-aft  axis  of  the  hend.  No  3. — The  eye 
looking  as  in  No.  2,  but  showing  equal  similar  inclinations  of  both  meridians  to  the  patient's 
right  with  reference  to  the  fore-and-aft  axis  of  the  eye.  No.  4. — Mode  of  reading  same. 


44  Tests  and  Studies  of  the  Ocular  Muscles 

drawn  upon  it  and  fastened  to  a  knitting  needle  R  R  which  lies 
against  some  oblique  diameter,  about  which  it  can  rotate  as  axis. 

The  cardboard  is  transfixed  through  its  center  by  another 
knitting  needle  /perpendicular  to  its  plane  and  not,  therefore,  visible 
in  No.  I  except  as  a  round  spot  (being  seen  foreshortened)  at  /. 

From  the  extremity  of  this  needle  (compare  /  in  No.  2)  is 
suspended  a  small  weight  W  by  a  fine  thread. 

The  cardboard  is  to  represent  the  equatorial  plane.  The 
thread-bearing  needle  represents  the  "line  of  fixation,"  and  is, 
therefore,  perpendicular  to  the  plane,  under  all  circumstances. 

Mode  Of  Use. — First  adjust  the  whole  as  in  the  primary  position 
of  the  eye  (No.  i,  Fig.  14).  The  cardboard  will  be  in  a  vertical 
transverse  plane  and  the  line  of  fixation  will  look  straight  forward. 

Let  the  anterior  extremity  /  of  the  thread-bearing  needle  repre- 
sent the  "point  of  fixation,"  and  the  thread  itself  be  the  ever- 
vertical  line  passing  through  the  fixation  point. 

Now,  let  the  observer  hold  his  eye  so  as  to  be  in  a  line  with 
the  thread-bearing  needle  ;  its  extremity  will  then  hide  its  length 
from  view,  and  the  appearance  will  be  that  presented  in  No.  I. 
The  thread  will  appear  to  coincide  with  the  vertical  meridian  V  V 
of  the  cardboard,  showing  that  there  is  no  torsion. 

To  proceed  to  the  next  step  :  since  any  oblique  motion  of  the 
eye  from  the  primary  position  must  take  place  about  an  axis  in 
Listing's  plane,  let  R  R  be  that  axis,  and  rotate  the  card,  as  in 
No.  2,  just,  as  the  equatorial  plane  of  the  eye  would  be  rotated 
actually.  Let  us,  for  instance,  make  the  eye  look  upwards  and  to 
the  (patient's)  right,  as  in  the  figure.  The  thread-bearing  knitting 
needle,  which  represents  the  line  of  fixation,  now,  therefore,  points 
upwards  and  to  the  right.  While  it  continues  to  do  so,  notice  that, 
if  your  own  eye  is  still  in  the  same  position  as  before,  the  appear- 
ance presented  is  that  of  No.  2. 

From  this  point  of  view,  the  vertical  meridian  V  V  no  longer 
appears  parallel  to  the  vertical  thread,  but  slopes  towards  the 
patient's  left,  as  if  to  indicate  laevotorsion  of  the  eye. 

The  horizontal  meridian,  on  the  other  hand  (h  //),  appears 
tilted  from  the  horizontal  in  exactly  the  opposite  direction,  as  if  to 
indicate  dextrotorsion  of  the  eye. 

Moreover,  the  apparent  laevotorsion  indicated  by  the  tilt  of  the 
vertical  meridian  is  exactly  equal  to  the  apparent  dextrotorsion 
indicated  by  the  tilt  of  the  horizontal  meridian. 


The  Ocular  Motions  45 

From  this  point  of  view,  therefore,  namely,  from  directly  in 
front  of  the  patient's  face,  there  is  no  torsion  whatever.  This 
proves  what  we  have  already  said,  that  the  eye  is  deprived  of  one 
degree  of  freedom,  and  that  rotation  about  any  axis  in  Listing's 
plane  cannot  have  a  component  about  a  line  perpendicular  to 
that  plane. 

Are  we  to  conclude  that  there  is  no  false  torsion,  then  ?  By 
no  means  ;  but  we  must  look  at  the  eye  from  the  true  point  of 
view  to  see  it,  namely,  along  the  length  of  its  own  visual  axis,  as 
already  specified. 

Let  us  do  so.  Leaving  the  model  as  in  No.  2,  let  the  observer 
move  his  own  eye  till  he  looks  along  the  line  of  the  knitting 
needle  /,  and  now  it  is  evident  that  both  meridians  tell  the  same 
story,  for  they  are  perpendicular  to  each  other  and  both  indicate 
that  the  eye  is  dextrotorted.  This  is  illustrated  in  No.  3. 

The  amount  of  dextrotorsion  can  be  read  off  from  the  graduated 
arc  on  the  cardboard  disk  by  seeing  what  degree  appears  crossed 
by  the  weighted  thread.  To  facilitate  doing  this,  the  eye  may  be 
held  lower  down  in  the  same  vertical  plane,  taking  care,  as  in 
No.  4,  to  keep  the  thread  in  apparent  coincidence  with  the  needle 
it  hangs  from. 

A  pretty  way  of  demonstrating  to  others  is  to  hold  a  strong 
light  in  such  a  position  as  to  make  the  shadow  of  the  thread  pass 
through  the  center  of  the  card,  as  in  No.  4.  This  linear  shadow 
will  then  record  the  amount  of  torsion  on  the  scale.  The  light 
should  be  held  on  a  slightly  lower  level  than  the  center  of  the 
cardboard  disk. 

For  greater  accuracy,  a  rather  better-made  apparatus  is  de- 
sirable. In  my  own  model  the  card  is  replaced  by  a  graduated 
circle,  of  some  white  material,  like  ivory  ;  and  is  pivoted  at  its 
center  to  the  oblique  axis,  so  that  this  can  be  brought  to  coincide 
with  any  required  diameter,  and  the  axis  itself  is,  by  a  graduated  half- 
circle,  capable  of  adjustment  to  any  required  degree  of  obliquity. 
Moreover,  the  degree  of  rotation  imparted  to  the  cardboard  disk  is 
registered  by  a  small  graduated  circle  (S  S  in  No.  i)  fixed  on  the 
oblique  axis  perpendicular  to  it. 

If  accurately  made,  such  a  torsion  calculator  at  once  tells  us 
the  amount  of  secondary  torsion  which  belongs  to  any  motion  of 
the  eye  from  the  primary  position  about  no  matter  what  axis  or  to 
what  extent. 


46  Tests  and  Studies  of  the  Ocular  Muscles 

A  very  little  experimenting  witu  this  apparatus  will  easily 
show  the  reader  that  if  Listing's  law  be  true  the  following  facts  are 
also  true  for  binocular  parallel  movements  of  the  eyes  : 

(a)  Fixation  upwards  and  to  the  right  is  accompanied  by 
parallel  dextrotorsion. 

(^)  Fixation  downwards  and  to  the  right,  by  parallel  laevotorsion. 

(f)   Fixation  upwards  and  to  the  left,  by  parallel  laevotorsion. 

(a?)  Fixation  downwards  and  to  the  left,  by  parallel  dextrotorsion. 

Clinical  Import. — Though  of  great  physiological  interest,  the 
clinical  importance  of  secondary  torsion  and  of  Listing's  law,  which 
it  expresses,  is  very  small.  It,  or  rather  the  unnatural  absence  of 
it,  accounts  for  the  obliquity  of  the  false  image  in  paralyses  of  the 
internal  and  external  recti,  during  diagonal  fixation  ;  and  when  a 
strong  paralysis  of  one  of  these  muscles  is  complicated  by  a  feeble 
paresis  of  another  muscle,  the  proper  paretic  torsion  due  to  the 
slight  paresis  might  conceivably  be  overborne  by  the  false  torsion 
in  the  opposite  sense. 

Geometry  of  False  Torsion.— The  torsion  calculator  makes  it  almost 
unnecessary  to  lead  the  reader  into  an  analytical  study  of  false  torsion,  and 
I  will  simply  show  how  to  obtain  my  formula. 

To  start  with,  we  will  assume  Listing's  law  proved  and  suppose  the  eye 
to  be  in  the  primary  position  before  the  motion  commences.  Required  :  for 
any  given  rotation  about  any  given  diameter  in  Listing's  plane,  to  find  the 
amount  of  "false  torsion." 

It  will  serve  our  convenience  best  to  select,  not  the  vertical  meridian  of 
the  retina,  or  the  vertical  meridian  of  the  cornea,  by  which  to  gage  the 
amount  of  torsion,  but  that  diameter  of  Listing's  plane  in  which  it  is  inter- 
sected by  the  plane  which  passes  through  the  vertical  meridian  of  the  cornea 
and  the  retina  and  which,  therefore,  is  itself  strictly  vertical  in  the  primary 
position  of  the  eye.  This  diameter  corresponds  to  the  line  V  V  on.  the 
cardboard  model  of  Listing's  plane  in  No.  i  of  Fig.  14.  We  wish  to  ask, 
for  any  position  of  the  eye :  What  is  the  angle  included  between  this  line 
and  a  vertical  plane  passing  through  the  center  of  motion  of  the  eyeball, 
and  the  point  of  fixation  ? 

It  is  evident  that  if  the  eyeball  were  free  to  rotate  unhinderedly  about 
any  oblique  axis,  this  vertical  diameter  of  Listing' s  plane  (as  we  may 
call  it)  would  describe  two  right  cones  with  their  vertices  .meeting  at  the 
center  of  motion  of  the  eye,  and  they  would  be  what  are  called  "opposite" 
cones,  since  they  have  one  axis  in  common,  namely,  the  axis  of  rotation 
of  the  eye.  Let  Fig.  15  represent  one  of  these  cones,  C  being  the  center 
of  motion  of  the  eyeball,  O  C  the  axis  of  rotation  and  V  C  the  vertical 
diameter  of  Listing's  plane  during  the  primary  position  of  the  eye,  while 
C  n  is  the  same  line  after  a  given  amount  of  rotation  ;  V  C  is,  in  fact,  the 
generating  line  of  the  cone. 


The  Ocular  Motions 


47 


The  triangle  V ' S  C  lies  in  Listing's  plane,  and  the  angle  formed  between 
it  and  the  plane  n  O  C  measures  the  rotation  of  the  globe  about  the  axis 
O  C.     Let  us  denote  this  angle  of   rotation  («  O  V),  whose  arc  is  n  F,  by 
the   letter  R ;    and    let  /  denote    the 
angle  O  C  V  by  which  the  axis  of  rota- 
tion O  C  is  inclined  to  the  vertical. 

It  is  this  angle,  indeed,  by  which  we 
define  the  axis  of  rotation,  for  there  are 
an  infinite  number  of  diameters  in  List- 
ing's plane  about  which  the  eyeball 
might  rotate,  but  only  one  for  each 
specified  angle  from  the  vertical,  though 
we  need  to  take  account  of  whether  the 
inclination  is  positive  (to  the  patient's 
right)  or  negative  (to  the  patient's  left). 

From  V,  drop  the  perpendicular 
Vm  upon  O  n. 

Then  m  Cn  gives  us  the  angle  of 
false  torsion  required  ;  for  n  C  is  the 
position  of  the  generating  line  at  the 
close  of  the  rotation,  and  shows  the 
new  position  of  the  vertical  diameter  of 
Listing's  plane,  while  the  plane  m  C  V 
is  the  vertical  plane  passing  through  the 
center  of  motion  and  the  point  of  fixa- 
tion, the  angle  between  these  two  being 
the  angle  of  torsion. 

It  is  evident  that  the  plane  m  C  V 
is  a  vertical  plane,  since  it  passes 
through  the  vertical  line  V  C. 

It  is  equally  easy  to  prove  that  the 
plane  m  C  V,  if  prolonged,  would  pass  through  the  fixation  point,  for  it  is 
perpendicular  to  the  plane  n  O  C,  to  which  the  line  of  fixation  is  also  of 
necessity  perpendicular,  and  they  both  pass  through  C;  therefore,  the  line 
of  fixation  must  lie  in  the  plane,  and  conduct  it,  so  to  speak,  to  the  fixa- 
tion point. 

Taking     V  C  as  unity— 
Since  O  V  =  Sin.  I 

Om 


Fig 


Design  proposed  by  author  for  solving  false 
torsion.  C. — Center  of  Motion  of  the 
eye.  VS  C. — Plane  passing  through 
C,  perpendicular  to  the  Visual  axis. 
n  O  C. — Same  Plane  after  Rotation  of 
eye  Up  and  to  the  Right  about  an  ob- 
lique axis  O  C.  Vm  C, — Vertical  plane 
through  V  C,  perpendicular  to  n  O  C. 
While  n  C  was  the  Original  Vertical 
Diameter  of  the  eye  in  the  Primary 
Position  (since  before  rotation  it  coin- 
cided with  VC},m  C  is  the  New  Verti- 
cal Diameter — the  angle  between  them 
(n  Cm)  being  the  angle  of  Torsion. 


and 


Moreover, 


But, 


Or, 


O 


= 


O  m  —  Sin.  I  Cos. 

O  C  =  Cos.  I 

O  m  _  Sin.  I  Cos. 

~o~c~ 

Om 


R. 


R 


Cos.  I 
=  Tan.  (I  —  x] 


=    Tan.  I  Cos.  R. 


O  C 

Tan.  (  I  —  x).=  Tan.  I  Cos.  x. 

x  =  I  —  Tan."1  (Tan-  !  Cos-  R 


48 


Tests  and  Studies  of  the  Ocular  Muscles 


Putting  this  into  language  : — The  false  torsion  is  equal  to  the  angle  from 
the  vertical,  or  from  the  horizontal,  of  the  axis  about  which  the  eye  rotates, 
less  the  angle  whose  tangent  is  the  multiple  of  the  tangent  of  the  inclination 
of  the  axis  of  motion  with  the  cosine  of  the  angle  traversed  by  the  line  of 
fixation. 

The  following  short  table  will  give  an  idea  of  the  amount  of  false  torsion 
which  takes  place  on  looking  in  any  diagonal  direction  midway  between  any 
two  of  the  cardinal  directions. 

Since  the  greatest  false  torsion  of  which  the  eye  is  capable  occurs  at  the 
extremities  of  these  diagonals,  we  may  see  at  once  that  it  does  not  ever 
much  exceed  10°. 

ROTATION    ABOUT   AN   AXIS   45°    FROM    THE    HORIZONTAL. 


Degrees 

5° 

10° 

15° 

20° 

25° 

30° 

35° 

40° 

45° 

Torsion 

e#' 

26' 

i° 

i°47' 

2°49' 

4°6/ 

5°4t/ 

7°33/ 

9°44' 

Azimuth  and  Altitude. — The  ocular  motions  can,  for  exact  work,  be 
analyzed  with  reference  to  three  principal  axes,  a  vertical  axis,  a  horizontal 
axis  and  an  antero-posterior  axis. 

When  the  eye  looks  directly  upwards  or  downwards  it  rotates  round  a 
horizontal  (or  transverse)  axis. 

When  it  looks  directly  to  the  right  or  left,  it  rotates  round  a  vertical  axis. 
These  will  be  recognized  as  the  cardinal  movements  of  the  eye. 
In  astronomical  language,  we  might  call  the  upward  and  the  downward 
motion,  "motion  in  altitude,"  and  the  motion  to  right  or  left,  "  motion  in 
azimuth,"  these   being  the  terms  that  would   be   used  were  the  eyes  two 
telescopes. 

Motion  in  azimuth  may  be  illustrated  by  that  of  a  weathercock  :  it  is 
motion  about  a  vertical  axis, 

Motion  in  altitude  may  be  illustrated  by  a  piece  of  cannon,  or  by  a 
toilet  looking  glass  :  it  is  motion  about  a  horizontal  axis. 

It  will  be  seen  that  the  cardinal  motions  of  the  eyes  are  those  of  either 
pure  azimuth  or  pure  altitude. 

When  the  visual  axis,  however,  is  directed 
obliquely  to  an  object,  altitude  and  azimuth  are 
combined.  What  is  so  wonderful  is  that  they 
are  combined  in  the  same  proportion  at  every 
instant  during  the  motion,  so  that  the  visual  axis 
instead  of  first  moving  sideways,  and  then  up 
and  down,  moves  at  once  by  the  shortest  route 
into  its  new  position. 

An  astronomer  would  direct  his  telescope  by 
KIK.  KJ  first  moving  it  in  azimuth  and  then  in  altitude,  but 

To  show  how  the  eye  reaches    this  is  far  too  clumsy  a  plan  for  the  eye,  since  it 
anv  new  position  hv  the  .  ,      , 

shortest  possible  route.        means  two  motions  instead  of  one,  and  a  longer 


The  Ocular  Motions 


49 


route  instead  of  the  shortest.  The  visual  axis,  therefore,  sweeps  along  what- 
ever inclined  plane  is  common  to  its  initial  position  and  its  new  position,  and 
loses  no  time  (Fig.  16).  It  is  evident  that  in  motion  of  this  kind  the  globe 
must  rotate  about  an  axis  perpendicular  to  this  inclined  plane,  an  axis, 
therefore,  which  is  neither  horizontal  or  vertical,  but  somewhere  inter- 
mediate. All  the  same,  it  can  be  described  in  terms  of  its  component 
azimuth  and  altitude  as  if  it  had  reached  its  new  position  like  a  telescope. 
The  horizontal  component  of  the  motion  is  the  azimuth,  and  its  vertical 
component  the  altitude. 

When  motion  is  to  the  right  from  the  initial  position,  the  azimuth  is  by 
astronomers  called  positive — when  to  the  left,  negative. 

Similarly,  motion  upwards  gives  positive  altitude,  and  motion  down- 
wards negative. 

In  analyzing  any  motion,  it  is  a  good  plan  to  adhere  to  the  rule  of 
allowing  azimuth  the  first  place,  or  preference,  over  altitude,  so  that,  for 
instance,  a  motion  of  ( —  20°  -\-  10°)  means  that  there  is  negative  azimuth 
of  20°  with  positive  altitude  of  10°,  or,  in  other  words,  the  eye  looks  20°  to 
the  left  and  10°  upwards. 

For  ordinary  clinical  work,  however,  it  is  well  to  substitute  for  motion  in 
azimuth,  motion  "to  right  and  left"  (dextroduction  and  lo'voduction),  which 
leaves  it  an  open  question  whether  it  is  about  an  axis  strictly  vertical,  or  with 
an  inclination  forwards  or  backwards.  For  motion  in  altitude,  elevation  and 
depression  are  suggested  as  terms  which  do  not  bind  us  too  closely. 


Fig.   17 

Varying  altitude  (to 
illustrate  torsion- 
less  motion  ac- 
cording to  Helm- 
holtz). 


is 

Constant  altitude  (to 
illustrate  torsion- 
less  motion  ac- 
cording to  Bon- 
ders). 


Helmholtz's  Plan  of  analyzing  the  ocular  motions  was  to  consider  the 
fixation  plane  (in  which  both  the  fixation  lines  lie)  as  first  elevated  or 
depressed,  above  (brow-wards)  or  below  (chin-wards)  its  "initial  position," 
by  an  angle  called  the  "elevation  angle"  of  fixation.  Then,  in  this  plane, 
the  angle  between  its  mesial  line  and  the  fixation  line  was  called  the  side- 
turning  angle.  By  this  plan,  however,  the  altitude  of  the  fixation  line 
steadily  lessens  as  the  lateral  deviation  increases,  and  it  was  partly  its 
adoption  which  led  to  the  apparent  discrepancy  between  Helmholtz's  laws 
of  false  torsion  and  those  in  the  text-books.  It  may  be  illustrated  in  a 
simple  way  by  a  weathercock  with  a  bent  stem,  as  in  Fig.  17,  where  motion 
in  azimuth  and  in  altitude  are  compounded.  Fig.  19  illustrates  pure  motion 
in  azimuth,  and  Fig.  18  motion  in  azimuth  with  a  constant  altitude,  as  in 
Donders'  plan. 


50  Tests  and  Studies  of  the  Ocular  Muscles 

Since  many  of  our  tests  are  conducted  with  the  patient  facing  a  flat  wall, 
it  way  be  well  to  point  out  in  what  respects  the  two  plans  differ  with 
reference  to  such  a  plane  surface. 

By  Helmholtz's  plan,  horizontal  lines  on  the  wall  represent  lines  of 
elevation  of  the  visual  plane,  and  if  each  is  marked  in  tangents  of  degrees 
to  right  and  left  of  a  central  zero,  these  represent  the  amount  of  lateral 
deflection.  If,  however,  the  lateral  deflection  take  place  first,  during  the 
primary  position  of  the  fixation  plane,  then  elevation  and  depression  of  this 
plane  makes  the  fixation  line  describe  a  hyperbolic  curve  on  the  wall,  with 
its  concavity  outwards. 

By  the  other  plan,  lines  of  equal  altitude  on  the  wall  are  hyperbolic 
curves  with  their  concavity  upward  when  the  eves  are  elevated,  and  down- 


Fig.  20 

Horizontal  section  of  an  eye  abducted 
from  A  to  A',  to  show  the  author's 
conception  of  the  difference  be- 
tween the  laws  of  false  torsion 
formulated  by  Helmholtz  and  Bon- 
ders. 


Fig.  21 

Side  view  of  an  eye,  seen  in  (ortho- 
graphic) projection  against  a  ver- 
tical plane,  superducted  from  A  to 
A',  the  three  circles  being  pro- 
jected as  straight  lines,  to  illus- 
trate author's  conception  of  what 
would  be  the  path  of  no  torsion 
according  to  Bonders,  and  what 
would  be  the  path  of  no  torsion 
according  to  Helmholtz,  while  the 
actual  torsion  is  as  if  the  cornea 
pursued  the  intermediate  path 
towards  P. 


ward  when  depressed  ;  but  when  the  eyes  are  first  deflected  to  the  right  or 
left,  elevation  or  depression  makes  the  fixation  line  describe  vertical  lines 
on  the  wall. 

In  Figs.  20  and  21  I  have  represented  graphically  the  different  points  of 
view  taken  by  Helmholtz  and  Donders.  Fig.  20  is  a  horizontal  section  of 
an  eye,  viewed  from  above,  and  abducted  from  A  to  A' ' .  The  diameter 
which  I  have  named  "agreed  axis"  is  the  one  about  which  rotation  would 
produce  exactly  the  false  torsion  which  all  observers  are  agreed  upon. 

Of  the  two  authorities  in  question,  each  adopts  the  diameter  indicated 
by  his  name  as  the  axis  about  which  torsionless  rotation  would  take  place. 
Since  these  diameters  are  inclined  at  equal  angles  to  the  "agreed  axes," 
though  on  opposite  sides,  Helmholtz's  tables  hold  good  for  Donders'  plan 
if  only  the  signs  be  changed. 


The  Ocular  Motions  51 

Fig.  21  shows  a  side  view  of  an  eyeball,  not  in  section  but  solid,  though 
viewed  as  projected  on  to  a  vertical  plane  ;  to  demonstrate  that  for  any 
given  elevation  of  the  cornea  from  A  to  A',  the  path  of  zero  torsion 
adopted  by  such  authority  is  that  circle  indicated  by  his  name,  the  path 
which  would  give  the  actually  observed  torsion  bisecting  the  angle  between 
them.  In  fact,  we  may  say  that,  for  any  secondary  position  of  the  eye 
whatever,  the  false  torsion  is  the  same  as  if  the  cornea  had  reached  that 
position  by  vertical  motion,  and  then  through  an  arc  of  a  circle  passing 
through  the  center  of  the  cornea  (A'}  and  the  primary  position  in  space  (P) 
of  the  posterior  pole  of  the  eye.  It  will  not,  of  course  be  supposed  that 
the  cornea  actually  takes  this  path,  but  its  torsion  is  the  same  as  Af  it 
had  taken  it. 


CHAPTER   III 


Individual  Ocular  Muscles 

The  Laws  of  Motion  are  not  Explained  by  the  Anatomy  of 
the  Muscles. — Our  studies  of  the  ocular  motions  up  to  this  point 
have  been  quite  independent  of  the  ocular  muscles,  and  our  deduc- 
tions from  them  would  not  have  suffered  had  we  possessed  no 
knowledge  of  their  anatomy. 

No  sooner  do  we  investigate  the  musculature,  than  we  find  the 
remark  of  Helmholtz  to  be  true,  that  "The  manner  in  which  the 
eye  is  fixed  presents  no  obstacles  to  any  rotations  whatever  of  a 
moderate  amplitude  :  the  existing  muscles  would  suffice  equally  well 
to  rotate  the  eye  about  any  given  axis  whatever. ' ' 

But,  he  adds,  "  In  the  ordinary  circumstances  of  normal  vision 
the  eye  is  far  from  executing  all  the  movements  of  which  the 
mechanical  possibility  is  recognized."*  These  remarks  are  con- 
firmed to  a  considerable  extent  by  the  phenomena  of  paralysis  of 
isolated  ocular  muscles. 

The  Laws  Explained  by  Innervation. — The  limitation,  there- 
fore, of  the  parallel  ocular  motions  to  rotations  about  diameters 
perpendicular  to  the  visual  axis,  is  a  limitation  which  finds  no 
proper  explanation  from  anatomy,  but  is  due  almost  entirely  to 
cerebral  co-ordination. 

If  we  except  the  internal  and  external  recti,  any  one  of  the 
other  muscles,  acting  in  an  isolated  way,  would  rotate  the  eye 
about  an  axis  lying  far  out  of  the  perpendicular  ;  but,  as  a  matter 
of  fact,  they  never  do  act  in  an  isolated  way,  but  in  innervational 
conjunction  with  some  other  muscle  in  such  a  manner  that  the 
resultant  axis  is  perpendicular. 

Brief  Description  Of  the  Recti. — Each  eyeball  is  controlled  by 
four  recti  and  two  obliques.  The  recti  spring  from  an  oval  tendinous 
tube  at  the  apex  of  the  orbit.  Since,  however,  there  is  not  quite 
sufficient  room  for  their  origins  round  the  optic  foramen,  this  tube 
spans  the  sphenoidal  fissure  to  be  attached  to  the  well-known  spine 
on  its  lower  margin.  From  the  orbital  surface  of  this  common 
tendon,  so  as  least  to  affect  the  optic  nerve  by  their  contraction, 

*"  Optique  Physiologique,"  p.  598. 

52 


Individual  Ocular  Muscles  53 

spring  the  muscular  fibres  of  the  four  muscles,  those  of  the  superior 
rectus  being  almost  continuous  with  those  of  the  levator  palpebrae 
at  first,  though  separating  later,  while  the  internal  rectus  is  con- 
tiguous to  the  origin  of  the  superior  oblique.  From  the  upper 
span  across  the  sphenoidal  fissure,  as  well  as  from  the  spine  itself, 
springs  the  external  rectus,  while  the  lower  span  gives  rise  to  tht 
inferior  rectus. 

The  internal  rectus  proceeds  almost  straight  forwards  and  lies 
rather  close  to  the  slightly-convex  inner  wall  of  the  orbit. 

The  external  rectus  proceeds  forwards  and  outwards  to  the 
outer  side  of  the  globe. 

The  superior  and  inferior  recti  proceed  forwards  and  somewhat 
outwards  to  the  upper  and  lower  parts  of  the  globe  respectively. 

Spiral  Of  Insertions. — The  insertions  of  the  internal,  inferior, 
external  and  superior  recti  lie,  in  round  numbers,  five,  six,  seven 
and  eight  millimeters  respectively  from  the  corneal  margin  (Tillaux). 
The  internal  rectus,  therefore,  has  the  greatest  mechanical  advantage 
and  the  inferior  next. 

Description  Of  Insertions. — The  insertions  of  the  internal  and 
external  recti  form  two  perpendicular  lines,  so  that  their  tendons 
appear  to  have  rectangular  extremities.  The  tendon  of  the  internal 
rectus  is  strong  and  well  defined  :  that  of  the  external  is  thinner, 
its  margins  passing  almost  insensibly  into  the  lateral  expansions 
connected  with  Tenon's  capsule. 

The  superior  and  inferior  recti  have  oblique  insertions,  the 
outer  extremities  of  which  are  rounded  and  lie  farther  back  than 
the  inner  extremities.  In  operations  on  their  tendons  this  should 
be  remembered,  so  as  to  approach  them  on  their  inner,  more 
accessible,  side.  Otherwise  some  difficulty  may  be  experienced  in 
hooking  them  up. 

Relative  Strength. — Of  all  the  muscles,  the  internal  rectus  is 
the  strongest,  or  at  least  the  most  bulky,  weighing,  according  to 
Volkmann,  .747  of  a  gramme  ;  the  external  rectus  comes  next, 
weighing  .715  of  a  gramme  ;  the  inferior  rectus  weighs  .671,  and 
the  superior  rectus  (the  weakest  of  all)  .514  of  a  gramme. 

Description  Of  Obliques. — The  superior  oblique  arises  from  the 
medial  side  of  the  upper  part  of  the  origin  of  the  internal  rectus 
and  runs  forward  ;  but  its  tendon  is  reflected  in  a  fibro-cartilaginous 
pulley  or  "  trochlea "  near  the  upper  inner  corner  of  the  orbital 
outlet,  whence  it  passes  backwards  and  outwards  over  the  orbital 


54  Tests  and  Studies  of  the  Ocular  Muscles 

surface  of  the  superior  rectus  tendon  to  be  attached  to  the  globe  by 
a  flattened  expansion  mostly  in  the  upper  and  outer  quadrant  of  its 
posterior  hemisphere.  The  line  of  insertion  is  subject  to  con- 
siderable variation,  its  direction,  according  to  Fuchs,  being  more 
longitudinal  in  myopes.  In  emmetropes  it  forms  nearly  equal  angles 
with  the  antero-posterior  and  transverse  meridians  of  the  globe. 
Though  the  anatomical  origin  of  the  superior  oblique  lies  on  the 
apex  of  the  orbit,  the  pulley  may  be  regarded  as  its  virtual  origin. 

The  inferior  oblique  arises  from  a  little  depression  in  the  bone 
near  the  lower  and  inner  corner  of  the  orbital  outlet,  and  passes 
backwards  and  outwards,  beneath  the  orbital  surface  of  the  inferior 
rectus,  continuing  to  curve  round  the  globe  between  it  and  the 
external  rectus,  till,  without  a  tendon,  its  muscular  fibers  gain 
insertion  about  a  quarter  of  an  inch  from  the  tendon  of  the  superior 
oblique,  either  in  the  same  quadrant  or  between  the  upper  outer 
and  the  lower  outer  quadrants  of  the  posterior  hemisphere.  The 
line  of  insertion  is  not  parallel  to  that  of  the  superior  oblique. 

Why  a  Pulley  at  all? — Many  must  have  wondered  why  one 
of  the  oblique  muscles  should  have  a  pulley  and  the  other  not. 
There  must,  of  course,  be  a  reason,  and  the  following  con- 
sideration is  advanced  as  a  possible  one  : 

The  speed  with  which  a  muscle's  point  of  insertion  moves  is 
proportional  to  its  length.  By  this  it  is  not  meant  that  a  long 
muscle  takes  necessarily  a  different  time  to  attain  its  maximum  con- 
traction, from  a  short  one  ;  but  that  in  the  same  period  of  time  it 
will  move  its  insertion  through  more  space  than  a  short  one  and, 
therefore,  with  a  greater  speed  of  motion.  The  obliques,  therefore, 
must  have  a  certain  length  if  they  are  to  keep  pace  with  the  recti  ;  * 
and  this  length  could  not  be  afforded  were  they  both  to  have 
origins  similar  to  that  of  the  inferior  oblique,  unless,  indeed,  they 
passed  each  other  in  curling  round  the  globe,  which  would  spoil 
their  action,  for  it  is  advantageous  that  their  insertions  should  be 
opposite  each  other,  rather  than  side  by  side. 

To  secure  sufficient  length  for  its  muscular  belly,  the  inferior 
oblique  has  to  curl  round  more  than  its  own  share  of  the  globe, 
and  this  would  leave  the  superior  oblique  short.  In  fact,  to  gain 
more  space,  the  inferior  oblique  dispenses  with  a  tendon  of  insertion 
altogether,  its  muscular  fibers  extending  quite  to  its  attachment. 


*  Really,  the  insertions  of  the  obliques  move  more  slowly  than  those  of  the  recti  during 
most  movements  of  the  eves. 


Individual  Ocular  Muscles  55 

Why  the  Superior  Oblique  and  not  the  Inferior? — Why,  then, 
it  may  be  asked,  does  the  inferior  oblique  not  have  the  pulley 
instead  of  the  superior,  seeing  it  is  the  inferior  which  is  supplied  by 
the  third  nerve  and  thus  might  be  expected  to  rise  in  company  with 
the  third-nerve  recti,  rather  than  the  superior  oblique,  which  is 
supplied  by  the  fourth  nerve  ? 

The  reason  may  just  possibly  be  this  :  that  prolonged  looking 
downwards  is  more  important  for  daily  work  than  looking  upwards, 
a  view  which  is  confirmed  by  the  fact  that  the  continuous  down- 
ward excursions  of  the  eye  are  more  amply  provided  for  than  the 
upward  excursions. 

The  center  of  motion  of  the  eyeball  approximates  more  closely 
to  the  geometrical  center  of  the  eye  on  looking  downwards  than  on 
looking  in  any  other  direction,  showing  that  the  mechanical  resis- 
tance is  least  during  this  motion  and,  moreover,  the  eye  can  make 
a  more  extended  excursion  downwards  than  upwards.  The  superior 
oblique,  therefore,  which  is  a  subductor  of  the  globe,  might  be 
expected  to  have  the  most  advantageous  arrangement  accorded  to 
it.  A  free  muscular  belly,  even  though  complicated  with  a  pulley, 
is  perhaps  if  not  stronger,  at  least  more  delicately  efficient  than  a 
muscular  belly  which  traverses  tissues  in  contact  with  the  globe  and 
is  embraced  by  a  check-ligament  (Fig.  n).  If,  on  the  other 
hand,  both  muscles  had  pulleys,  the  pulley-complication  would  be 
doubled  unnecessarily.  * 

Everything  for  Speed. — In  some  other  parts  of  the  body  the 
muscles  are  constructed  for  strength  rather  than  speed,  but  with 
the  eyeball  everything  is  adapted  for  speed. 

The  greater  the  number  of  muscular  fibres  ranged  side  by  side 
the  stronger  is  a  muscle,  and  the  greater  the  number  arranged  end 
to  end  the  quicker  it  is.  The  bi-penniform  arrangement,  e.  g. ,  of 
the  rectus  femoris,  is  a  beautiful  example  of  adaptation  for  strength 
at  the  expense  of  speed,  the  muscular  length  being  considerably 
less,  and  the  muscular  breadth  being  considerably  more,  than  the 
actual  length  and  breadth  of  the  muscle  as  a  whole.  The  muscu- 
lar length  is  found  by  measuring  along  the  lines  of  fibres  from  one 
tendon  to  the  other.  There  is  no  such  marked  arrangement  as 
this  in  the  muscles  of  the  orbit,  where  speed  is  more  desirable 

*M;uithner  has  shown  that  in  paralysis  of  the  inferior  oblique  the  vertical  separation  of 
the  doulile  images  is  greater  than  in  paralysis  of  the  superior  oblique.  But  then  the  superior 
rectus  is  very  much  weaker  than  the  inferior  rectus,  and  Mauthner's  observation  may  enly 
show  that  the  difference  between  the  obliques  is  less  than  that  between  the  recti. 


56  Tests  and  Studies  of  the  Ocular  Muscles 

than  strength  and  where,   therefore,   a    certain    length   of   belly  is 
indispensable. 

Touching  Point. — 'The  point  at  which  a  muscle,  as  we  trace  it 
forwards,  first  touches  the  globe,  or  its  momentary  insertion,  is 
continually  changing  its  location  with  every  movement  of  the  eye. 
The  "arc  of  contact,"  therefore,  along  which  the  muscle  remains 
applied  to  the  globe,  and  which  extends  from  the  touching  point  to 
the  anatomical  insertion,  is  correspondingly  ever  varying  in  length. 
Its  variations,  I  believe,  however,  are  tempered  by  the  disposition 
of  Tenon's  fascia,  so  that  in  the  extreme  rotations  of  the  eye,  the 
arc  of  contact  is  not  abolished  so  quickly  as  calculations  based  on 
the  muscles  only  would  lead  us  to  expect. 

Thus,  in  the  case  of  the  internal  rectus,  it  is  easy  to  calculate 
that  the  arc  of  contact,  while  the  eye  looks  straight  forwards,  is  about 
36°  (from  the  center  of  motion),  yet  many  eyes  can  be  adducted 
50°  or  perhaps  even  60°  at  a  push,  and  it  is  not  likely  that  the  arc 
of  contact  ceases  at  36°  or  the  eye  would  be  tugged  at  and  dis- 
torted. The  "collarettes"  or  "  intra-capsular  ligaments,"  as  they 
are  called,  must  tend  to  bind  the  tendon  longer  to  the  globe  so  as 
not  to  let  them  so  soon  part  company. 

Terminology. — When  the  cornea  is  drawn  toward  the  temple 
the  eye  is  said  to  be  abducted ;  when  towards  the  nose,  adducted  ; 
when  raised,  we  may  call  it  elevated ;  when  lowered,  depressed. 

When  the  eye  is  twisted  about  its  own  axis  so  as  to  make  the 
cornea  revolve  like  a  wheel,  we  may  call  it  torted ;  intorted  when 
the  upper  segment  of  the  cornea  revolves  towards  the  nose,  and 
extorted  when  it  revolves  towards  the  temple.  I  have  found  it 
convenient  also  to  speak  of  dextroduction,  lacvoduction,  dextrotor- 
sion  and  laevotorsion. 

Prime  Muscular  Functions. — Each  eye  possesses  one  muscle 
pre-eminent  for  abduction — the  external  rectus  ;  another  for  adduc- 
tion— the  internal  rectus  ;  for  elevation — the  superior  rectus  ;  for 
depression — the  inferior  rectus  ;  for  intorsion — the  superior  oblique  ; 
for  extorsion — the  inferior  oblique. 

Subsidiary  Functions.— But  besides  these  "prime"  actions, 
each  muscle  has  "secondary"*  actions.  This  is  least  so  with  the 
internal  and  external  recti,  which  are  pure  adductors  and  abductors 
respectively,  except  when  the  eyes  are  elevated  or  depressed. 

•*At  Prof.  Savage's  suggestion,  I  have  altered  th<;  word  "subsidiary"  to  "secondary,"  as 
more  euphonious. 


Individual  Ocaiat  Ahiscles  57 

They  have  subsidiary  vertical  and  torsional  effects,  but  I 
believe  to  a  far  less  extent  than  has  been  supposed,  owing  to 
the  restraints  imposed  by  the  collarettes  and  Tenon's  fascia,  which 
make  the  tendons  share  to  some  extent  any  change  of  direction 
imparted  to  the  eye.* 

Medial  Origins  Of  Muscles. — With  regard  to  the  secondary 
effects  of  the  superior  and  inferior  recti  and  the  obliques,  we  may 
assist  the  memory  by  recollecting  that  all  the  ocular  muscles,  with- 
out exception,  spring  from  origins  nearer  the  median  plane  than 
their  insertions. 

Hence,  the  superior  and  inferior  recti,  being  inserted  into  the 
anterior  hemisphere  of  the  globe,  pull  it  nearer  the  median  plane, 
i.  e. ,  abduct  the  cornea  ;  whereas,  the  obliques,  being  inserted  into 
the  posterior  hemisphere  of  the  globe,  pull  their  insertions  towards 
the  median  plane,  z.  e. ,  abduct  the  cornea. 

Moreover,  in  consequence  of  the  same  medial  disposition  of 
the  muscular  origins,  those  muscles  which  proceed  to  the  upper 
hemisphere  of  the  globe  by  pulling  their  insertions  inwards  zwtort 
the  cornea  ;  and  those  which  proceed  to  the  lower  hemisphere  of  the 
globe,  since  they  also  draw  their  insertions  inwards,  ^.rtort  the  cornea. 

We  may  say  then  that  the  superior  muscles  cause  intorsion, 
and  the  inferior  muscles  extorsion  ;  the  obliques  abduction,  and 
the  recti  (superior  and  inferior)  adduction. 

Thus  the  superior  rectus,  besides  elevating  the  cornea,  intorts 
the  eye,  because  its  insertion  is  "superior,"  and  adducts  it  because 
it  is  inserted  into  the  anterior  hemisphere. 

The  inferior  rectus,  besides  depressing  the  cornea,  causes 
extorsion  (being  "inferior")  and  adduction  (being  inserted  into 
the  anterior  hemisphere  of  the  globe). 

The  superior  oblique  causes,  besides  its  proper  intorsion 
(from  being  "superior"),  depression  of  the  cornea  from  the  upper 
character  of  its  insertion,  and  abduction  (being  inserted  into  the 
posterior  hemisphere  of  the  globe). 

The  inferior  oblique  causes,  besides  its  proper  extorsion  (from 
being  "inferior"),  elevation  of  the  cornea  because  its  insertion  is 
inferior  and  its  origin  anterior,  and  causes  abduction  from  the 
posterior  character  of  its  insertion. 


*Thongh  the  reader  need  concern  himself  but  little  with  these  unimportant  considera- 
tions, on  looking  up.  the  <//.///<;/  rn-lux  mu*t  l>e  a  slight  superductor  and  intortor.  and  on 
looking  down  a  slight  subductor  and  extortor.  On  looking  up,  the  internal  recttit  musl  he  a 
slight  superductor  and  extortor  ;  and  on  looking  down,  a  slight  subductor  and  intortor. 


58  Tests  and  Studies  of  the  Ocular  Muscles 

Inverse  Proportions  of  Prime  and  "  Secondary  "  Actions.— We 

now  come  to  a  point  of  considerable  clinical  importance. 

All  the  secondary  effects  of  the  various  muscles  which  we 
have  just  recounted  are  at  the  expense  of  their  prime  actions. 
The  energy  expended  in  producing  them  represents  so  much  loss 
in  the  prime  action  of  the  muscle. 

In  those  positions  of  the  globe,  therefore,  where  we  find 
secondary  effects  of  a  muscle  at  their  minimum,  the  prime  effect  is 
at  its  maximum,  and  vice  versa. 

Lateral  Superductors  and  Subductors. — The  superior  rectus  is 
a  "lateral"  sursumductor,  and  the  inferior  rectus  a  "lateral" 
dursumductor,  because  their  elevating  and  depressing  effect  is 
greatest  when  the  eye  is  sufficiently  abducted  towards  the  temple 
for  their  vertical  power  to  be  uncomplicated  and  their  secondary 
effects  to  become  practically  nil. 

The  more  the  eye  is,  on  the  other  hand,  adducted,  the  greater 
become  their  secondary  adducting  and  torsional  effects,  and  the 
less  efficient  they  are  for  the  vertical  movements  of  the  eye. 

When  we  come  to  the  diagnosis  of  ocular  paralysis  we  shall 
find  the  advantage  of  knowing  that  the  maximum  torsional  effect 
of  a  muscle  occurs  when  the  eye  looks  to  the  opposite  side  from 
that  in  which  its  maximum  vertical  effect  occurs. 

Medial  Superductors  and  Subductors. — The  obliques  have 
their  greatest  torsional  effect  when  the  eye  looks  outwards, 
because  then  they  form  the  greatest  angle  with  the  optic  axis, 
and  since  their  greatest  effect  on  the  vertical  motions  of  the  eye 
is  found  on  looking  towards  the  nose,  the  superior  oblique  is 
a  ' '  medial ' '  depressor,  and  the  inferior  oblique  a  ' '  medial ' ' 
elevator. 

While  the  reasons  for  these  facts  are  no  doubt  self-evident, 
their  fuller  consideration  requires  a  study  of  the  ' '  muscular  planes ' ' 
and  ' '  muscular  axes. ' ' 

Lines  Of  Force. — Owing  to  the  existence  of  an  "  arc  of  con- 
tact," the  muscular  forces  acting  on  the  globe  must  be  tangential 
forces.  There  is,  therefore,  one  tangent  line  to  the  globe  for  each 
muscle  which  indicates  its  direction  of  force. 

In  the  primary  position  of  the  eye  the  lines  of  force  probably 
extend  from  the  "touching  points"  of  the  several  muscles  to 
their  orbital  origins,  or  the  trochlea  in  the  case  of  the  superior 
oblique. 


Individual  Ocular  Muscles  59 

When  the  eye  is  moved  away  from  the  primary  position,  the 
lines  of  force  are,  I  believe,  more  or  less  diverted  by  Tenon's  fascia 
acting  as  a  pulley,  or  at  least  exerting  an  elastic  side-traction  where 
the  ' '  collarettes  ' '  exist. 

Muscular  Planes. — When  we  have  a  force  acting  tangentially 
on  a  rotating  body,  confined  as  by  a  center  of  motion,  and  the  line 
of  that  force  is  given,  the  plane  of  force  is  evident  at  once.  It  is 
the  plane  which  passes  through  the  line  of  force  and  the  center  of 


Fig.  22 

The  Muscular  Planes  and  Axes.     0— The  muscular  planes,  and  o  o'  the  axes  of  the  obliques. 
R — The  muscular  planes,  and  r  r'  the  axes  of  the  recti. 


motion,  and  in  the  case  of  the  eye,  is  called  a  "muscular  plane," 
there  being  one  muscular  plane  for  each  muscle. 

The  muscular  planes,  therefore,  all  agree  in  passing  through 
the  center  of  motion,  but  differ  in  that  each  extends  thence  through 
the  line  of  force  of  its  own  muscle. 

Three  Pairs. — The  muscular  planes  of  the  internal  and  exter- 
nal recti  are  practically  horizontal  and  identical. 

Those  of  the  superior  and  inferior  recti  are  generally  also 
supposed  to  be  identical,  but  vertical,  forming  an  angle  with  the 
median  plane,  which  is  estimated  by  Landolt  at  27°  in  accordance 
with  the  well-known  anatomical  fact  that  these  recti,  instead  of 


60  Tests  and  Studies  of  the  Ocular  Muscles 

running  directly  forwards,  run  forwards  and  somewhat  outwards  to 
their  insertion. 

In  a  horizontal  section  of  the  eye,  as  shown  in  Fig.  22,  their 
common  muscular  planes  are  represented  in  section  by  the  lines  R. 

The  muscular  planes  of  the  obliques  are  similarly  supposed  to 
be  identical  and,  therefore,  vertical,  but  running,  of  course,  back- 
wards and  inwards  instead  of  forwards  and  outwards,  so  as  to  form 
an  angle  with  the  median  plane  estimated  by  Landolt  at  51°. 

In  Fig.  22  the  common  muscular  planes  of  the  obliques  are 
represented  in  section  by  the  lines  O. 

Properties  of  Muscular  Planes.— Each  muscular  plane  possesses  this 
property,  that  if  a  single  muscle  were  to  contract,  the  mid  point  oi  its  insertion 
into  the  sclerotic  would  not  move  out  of  the  plane,  and  though  all  the  points 
contained  in  the  plane  would  move,  the  plane  itself  remains  fixed  in  space. 

Every  muscular  plane,  since  it  passes  through  the  center  of  motion, 
must  approximately  bisect  the  eye  and  cut  its  surface  in  a  great  circle.  All 
points  on  the  surface  of  the  eyeball  not  lying  in  this  circle,  will,  when  the 
muscle  contracts,  also  move  in  smaller  parallel  circles,  and  the  greater  the 
distance  of  each  surface  point  from  the  muscular  plane,  the  smaller  the 
circle  in  which  it  moves. 

When  the  center  of  the  cornea,  therefore,  lies  actually  in  the  muscular 
plane  of  a  muscle,  as  it  does,  for  example,  in  that  of  the  superior  rectus, 
when  the  eye  is  abducted  27°,  the  motion  of  the  cornea  under  the  influence 
of  that  muscle  is  greatest,  and  it  becomes  less  and  less  the  more  the  cornea 
is  moved  away  from  the  muscular  plane  by  adduction  of  the  eye. 

We  have  seen  that  the  parallel  circles  of  motion  on  the  surface  of  the 
globe  become  smaller  and  smaller  as  they  lie  farther  and  farther  from  the 
muscular  plane. 

This  goes  on  till  at  last  they  are  reduced  to  a  point  on  each  side  which 
remains  fixed  in  space  while  the  globe  rotates. 

These  points,  being  those  on  the  globe's  surface,  which  are  farthest 
removed  from  the  muscular  plane,  are  the  poles  of  the  "muscular  circle," 
and  the  diameter  of  the  globe  which  unites  them  and  which,  therefore, 
remains  fixed  in  space  with  them,  is  the  "axis  of  rotation"  and  is  strictly 
perpendicular  to  the  muscular  plane. 

Axis  Of  Rotation. — To  find  the  axis  of  rotation  for  any  muscle, 
we  must  first  find  the  muscular  plane  ;  then  a  line  perpendicular  to 
it,  and  passing  through  the  center  of  motion  is  the  axis  of  rotation. 
In  other  words,  the  muscular  plane  cuts  the  surface  of  the  globe  in 
a  circle,  the  axis  of  which  is  the  axis  of  rotation.* 

Fig.  22  shows  the  axes  of  rotation  as  usually  figured. 

*  Though  it  is  usual  in  opnthalmolo^y  to  speak  of  the  axis  of  rotation  as  a  diameter,  yet  it 
is  defined  physically  l>y  the  ratlins  perpendicular  to  the  muscular  plane,  on  that  side  of  it 
which  represents  the  sense  in  which  the  contract'ion  of  the  muscle  makes  the  eyeball  rotate. 


Individual  Ocular  Muscles  61 

Since  the  superior  and  inferior  recti  are  supposed  to  have  one 
muscular  plane  in  common,  they  have  a  common  axis  of  rotation 
(r  r' )  about  which,  however,  they  rotate  the  eye  in  opposite  senses. 

The  obliques  are  also  supposed  to  have  a  common  horizontal 
axis,  shown  by  o  o'  in  the  same  figure. 

Are  the  Axes,  Supposed  to  be  Identical,  Really  So  ?— Let  us  now  ask : 

Do  the  superior  and  inferior  recti  really  rotate  the  globe  about  one  axis  in 
common?  I  am  not  sure  that  they  do,  for  were  it  so,  isolated  paralysis  of 
either  muscle  would  (cceteris  paribus),  during  the  primary  attitude  of  the 
sound  eye,  adduct  the  other,  so  as  to  cause  homonymous  diplopia,  and  this 
is  contrary  to  most  recorded  clinical  experience.* 

Similarly,  it  is  doubtful  if  the  two  obliques  rotate  the  globe  about  an 
axis  common  to  them  both,  for  if  this  were  true,  paralysis  of  either  would 
tend  to  cause  crossed  diplopia  during  the  primary  attitude  of  the  sound  eye, 
which  is  also  contrary  to  usual  clinical  experience. 

It  is  true  that  sometimes  crossed  diplopia  does  occur  in  paralysis  of  the 
obliques,  but  this  has  hitherto  been  explained,  on  Mauthner's  hypothesis,  by 
the  liberation  of  previously-existing  latent  divergence  (exophoria)  of  the 
two  eyes. 

The  reader  may  feel  inclined  to  object:  "Surely,  if  contraction  of  a 
muscle  causes  adduction  (as  we  know  to  be  the  case  with  the  superior  and 
inferior  rectus),  its  paralysis  will  result  in  abduction  !" 

This  is  true  when  the  sound  eye  is  raised  or  lowered  towards  the  area 
of  maximum  diplopia,  for  the  superior  rectus  is  an  adductor  when  it  super- 
ducts  the  eye,  and  the  inferior  rectus  when  it  subducts  the  eye  ;  but  what  we 
are  now  considering  is  what  they  do  during  the  primary  position  of  the 
sound  eye. 

Double  Contraction.— It  it  were  possible  for  both  the  superior  and  the 
inferior  rectus  to  contract  simultaneously,  the  effect  on  the  rotation  of  the 
eye  in  its  primary  position  would  be  nil  if  they  really  have  but  one  axis  in 
common.  They  would  simply  tend  to  draw  the  eye  as  a  whole  backwards 
and  slightly  inwards  towards  the  apex  ot  the  orbit,  just  as  if  an  elastic 
string  were  tied  by  one  end  to  the  optic  foramen,  and  by  the  other  end  (if 
possible)  to  the  center  of  motion  ot  the  eye.  The  eye,  therefore,  would  be 
neither  adducted"  nor  abducted. 

Single  Contraction. — If,  however,  either  muscle  contracted  alone,  the 
vertical  motion  of  the  cornea  would  be  accompanied  by  a  proportionately 
increasing  adduction. 

Single  Paralysis. — So  far  the  reader  will  readily  agree  ;  but  the  next 
proposition  is  quite  as  simple  :  that  if  either  muscle  were  paralyzed,  the 
paralytic  displacement  of  the  cornea  should  (during  the  primary  position  of 
the  sound  eye)  be  accompanied  with  a  precisely  corresponding  proportion 
of  adduction  also.  For  if  the  ax-is  be  common  to  both,  paralysis  of,  say, 
the  superior  rectus,  must  cause  the  same  effects  as  contraction  of  the  inferior. 

*Since  this  proposition  requires  a  good  deal  of  thinking  out  before  becoming  evident  to 
every  reader,  the  smaller  print  may,  with  advantage,  be  skipped  on  first  reading. 


62  Tests  and  Studies  of  the  Ocular  Muscles 

When  the  eyeball  is  in  equilibrium,  each  tension  is  balanced  by  the 
resultant  of  all  the  other  tensions,  and  if  the  increase  of  any  one  tension 
rotates  the  eyeball  about  a  given  axis  the  resultant  of  all  the  other  tensions 
would,  in  the  absence  of  that  one,  rotate  the  eyeball  about  the  very  same 
axis,  only  in  the  opposite  sense.  But  this  is  just,  in  kind,  if  not  in  degree, 
what  the  other  rectus  does  when  it  contracts,  for  it  also  rotates  the  eyeball 
about  this  same  axis  in  the  opposite  sense.* 

Arc  Of  Contact. — This  name  has  already  been  spoken  of  as 
given  to  the  line  along  which  a  muscle  and  its  tendon  embrace 
the  surface  of  the  globe. 

The  actual  insertions  of  the  tendons  are  in  advance  of  the 
points  where  the  muscles  first  reach  the  surface  of  the  globe 
tangentially. 

The  touching  points  are  those  we  must  take  account  of  in 
studying  the  dynamics  of  the  ocular  muscles,  but  we  really  know 
much  less  about  them  than  is  usually  taken  for  granted,  since  they 
are  modified  by  Tenon's  capsule  in  a  way  which  it  is  impossible  to 
determine  precisely. 

This  shows  it  to  be  all  the  more  judicious  not  to  study  the 
ocular  muscles  synthetically,  i.  e. ,  by  argument  from  their  anatomy, 
but  analytically,  by  close  observation  of  the  actual  results  of  their 
physiological  action  and  pathological  failures.  Bonders  placed 
great  emphasis  on  this  principle. 

Ophthalmotropes. — For  teaching  purposes  the  synthetical  study 
is,  however,  needful  and,  provided  we  confine  ourselves  only  to 
broad  principles,  we  shall  not  go  far  astray. 

A  number  of  ophthalmotrope<>  have  been  invented  from  time  to 
time,  and  of  these  I  prefer  Landolt's  and  Anderson  Stuart's  as  the 
best.  Their  purpose  is  to  represent,  in  the  form  of  a  model,  the 
characteristic  functions  of  the  several  muscles  in  isolated  action,  as 
in  Figs.  23  and  24. 

Landolt's  Ball. — Another  ingenious  device  by  Landolt  is  his 
"india-rubber  ball"  (Fig.  25),  which  any  reader  can  easily  mark 
for  himself. 

His  own  description  is  as  follows  :  ' '  Take  a  simple  india-rub- 
ber ball,  depict  upon  it  the  cornea,  the  vertical  meridian  and  the 
horizontal  meridian.  On  the  latter,  mark,  at  39°  from  the  anterior 

*This  amounts  to  saying  that,  during  the  primary  position  of  the  sound  eye,  the  paralysis 
of  the  superior  rectus  produces  the  same  effect  as  slight  spasm  of  the  inferior,  if  the  usual 
single-axis  hypothesis  be  true.  Clearly,  therefore,  either  it  is  not  true,  or  else  previous  clinical 
observations  in  the  primary  area  of  the  motor  field  have  been  misleading.  I  will  not  attempt 
to  say  which  is  the  case. 


Individual  Ocular  Muscles  63 

pole  (center  of  the  cornea),  the  anterior  extremity  of  the  axis  of 
the  obliques  ((9),  and  at  63°  on  the  opposite  side  (/?),  the  axis  of 
the  superior  and  inferior  recti. ' ' 


Fig.  23 

Landolt's  Ophthaliuotrope.     The  eye  is  seen  under  the  action  of  the  superior  oblique,  O  O 
being  the  axis  of  the  oblique. 


Now  suppose,  for  example,  we  wish  to  demonstrate  the  action 
of  the  superior  oblique,  we  reason  thus  :  this  muscle  makes  all  the 
points  of  the  cornea  describe  parts  of  parallel  circles  about  its  axis. 


64 


Tests  and  Studies  of  the  Ocular  Muscles 


Taking  a  pair  of  compasses,  therefore,  we  open  them  so  that 
one  of  the  points  shall  correspond  with  the  anterior  extremity  of  the 
axis  of  the  oblique  muscle,  the  other  to  the  center  of  the  cornea. 


Fig.  34 

Anderson  Stuart's  Model  of  the  Ocular  Muscles  (the  inferior  obliques  should  not  have  a  pulley) 

Keeping  the  first  point  fixed,  we  trace  with  the  other  the  circle,  of 
which  a  part  is  traversed  by  the  apex  of  the  cornea  under  the 
influence  of  the  contraction  of  the  superior  oblique. 

"If  we  wish  to  know  where  this  apex  of  the  cornea  is  found 
after  a  rotation,  for  example,  of  40°,  we  have  only  to  trace  a 
straight  line,  starting  from  the  anterior  extremity  of  the  axis  and 
forming  an  angle  of  40°  with  the  horizontal  (below  it  for  the 
superior  oblique,  above  it  for  the  inferior  oblique). 

"The  point,  O1  or  R',  where  this  line  meets  the  circle  indi- 
cates the  position  of  the  corneal  apex  which  corresponds  to  the 
required  rotation.  We  see  thus,  at  once,  in  what  direction  and  to 
what  extent  it  deviates  from  the  horizontal  as  well  as  from  the 
vertical. 

"As  for  the  slope  which  the  vertical  meridian  of  the  cornea 
will  have  acquired  at  the  same  time,  it  is  of  necessity  perpendicular 
to  the  line  which  we  have  just  traced  and  passes  through  the  point 
which  it  has  discovered  for  us  to  be  the  center  of  the  cornea.  That 
is  evident.  This  very  line  is,  in  short,  no  other  than  a  part  of  the 


Individual  Ocular  Muscles  65 

horizontal  meridian,  sloped  by  the  muscular  contraction  ;  it  is 
perpendicular  to  the  vertical  meridian. 

"  It  is  thus  that  in  our  figure  the  two  black  stripes  indicate  the 
inclination  impressed  upon  the  vertical  meridian  of  the  right  eye  by 
the  superior  oblique  O1  and  by  the  inferior  rectus  R' . 

'•  By  dropping  a  perpendicular  from  the  points  O'  and  R' 
upon  the  horizontal  meridian,  we  get  the  amount  of  depression 
(  O'  h  and  R'  /i)  produced  by  the  muscles  in  question. 

1 '  The  perpendicular  dropped  from  these  two  points  O"  and  R 
upon  the  vertical  meridian  correspond  to  the  amount  of  abduction 
( O1  V)  caused  by  the  oblique,  and  to  the  amount  of  adduction 
caused  by  the  rectus. ' ' 

With  Landolt's  Ball  the  Sound  Eye  is  Supposed  in  its  Primary 
Position. — It  should  be  remarked  that  Landolt's  ball  as  thus  made 
only  represents  the  truth  when  the  internal  and  external  recti  are 


Fig.   25 

Laortnlt's  Ball 


quiescent,  for  the  more  the  eye  is  adducted  by  the  internal  rectus, 
the  farther  is  the  anterior  extremity  of  the  oblique  axis  removed 
from  the  cornea,  making  the  arc  for  the  oblique  become  that  of  a 
larger  circle  and  the  arc  for  the  rectus  that  of  a  smaller  circle. 


66  Tests  and  Studies  of  the  Ocular  Muscles 

Conversely,  the  more  the  eye  is  abducted  by  the  external 
rectus,  the  smaller  becomes  the  circle  for  the  oblique,  till  perhaps  it 
becomes  nil,  showing  that  then  the  oblique  is  purely  torsional  in  its 
action  :  and  the  larger  becomes  the  circle  for  the  rectus,  till  at  last 
it  becomes  a  straight  line,  showing  that  then  the  rectus  is  a  super- 
ductor  or  subductor. 

Furthermore,  since  the  circles  on  Landolt's  ball  map  out  the 
paths  followed  by  the  apex  of  the  cornea  under  the  action  of  indi- 
vidual muscles,  and  since  it  is  based  on  the  approximation  that  the 
obliques  have  one  and  the  superior  and  the  inferior  recti  also  have 
one  horizontal  axis  in  common,  we  may  well  use  it  to  illustrate  that 


Fig    26 

To  show,  if  the  axes  are  tilted,  the  nature  of  the  tilting,  /.  R.  and  S.  R.  being  the  axes  of  the 
Superior  and  Inferior  Recti,  and  S.  O.  and  /.  0.  those  of  the  Superior  and  Inferior  Obliques. 


if  this  be  true,  paralysis  of  any  one  of  these  muscles  would  bring 
about  the  opposite  horizontal  condition  from  that  which  is  generally 
believed. 

For  if  contraction,  say,  of  the  superior  rectus,  move  the  apex 
of  the  cornea  in  a  circle  as  marked  on  the  ball,  its  paralysis  will 
move  it  in  the  same  circle  but  in  the  opposite  direction,  i.  e.,  just  as 
slight  contraction  of  the  inferior  rectus  would  move  it,  causing, 
therefore,  adduction  in  each  case. 

Tilted  Axes. — If  clinical  observation  shows  abduction  to  be  the 
undoubted  result  of  uncomplicated  paralysis  of  the  superior  or 
inferior  rectus  during  the  primary  position  of  the  sound  eye,  then 
the  axes  of  rotation  for  these  muscles  must  be  regarded  as  inclined 


Individual  Ocular  Muscles  67 

to  the  horizontal  in  opposite  directions  ;  the  axis  for  the  superior 
rectus  having  its  inner  end  lower,  and  that  for  the  inferior  rectus 
having  its  inner  end  higher,  than  the  horizontal  meridian. 

Similarly,  if  uncomplicated  paralysis  of  either  oblique  cause 
adduction,  under  similar  conditions,  the  axis  of  rotation  for  the 
superior  oblique  must  have  its  outer  end  higher,  and  that  for  the 
inferior  oblique  lower,  than  the  horizontal  meridian. 

I  have  represented  this  in  Fig.  26,  which  shows  an  india- 
rubber  ball  traversed  by  knitting-needles  to  represent  the  axes. 

As  a  matter  of  fact,  it  is  extremely  difficult  to  ensure  that  any 
paralysis  is  uncomplicated  by  previously-existing  latent  squint,  so 
that  clinical  records  are  very  little  to  be  trusted  on  this  point,  unless 
they  are  made  with  special  reference  to  it. 

The  theory  of  tilted  axes  is,  I  find,  far  from  new,  Meissner 
having  taught  them,  and  later  Continental  writers  having  owned 
them  theoretically  as  true,  though  deeming  the  convenient  approxi- 
mation of  horizontal  axes  sufficiently  accurate  for  practical  purposes, 
and  for  clinical  deduction  perhaps. 


CHAPTER    IV 


Associated  Muscles  in  a  Single  Eye 

Isolated  Contraction  of  Some  Muscles  Unknown.- -Our  study 
of  the  ocular  motions  in  Chapter  II  will  have  shown  us  that  isolated 
action  is  forbidden  to  at  least  four  of  the  muscles  of  each  eye,  since 
neither  the  superior  or  inferior  recti,  nor  the  obliques,  can  act  alone 
without  violating  Listing's  law,  by  which,  it  will  be  remembered, 
all  rotations  from  the  primary  position  are  forbidden  to  the 
healthy  eye  except  those  about  axes  in  a  vertical  plane  passing 
through  the  center  of  motion  of  the  eye,  perpendicular  to  the 
visual  line  in  its  primary  position.  But  neither  the  axis  for  the 
obliques  nor  that  for  the  superior  and  inferior  recti  lie  in  this 
plane  ;  therefore,  no  one  of  these  muscles  can  contract  without 
some  associated  muscle  acting  with  it  in  that  perfect  proportion 
required  to  keep  the  resultant  axis  in  this  inevitable  plane.  The 
rotations  which  individual  muscles  would  effect  severally,  have  to 
be  compounded  with  great  nicety  into  one  rotation. 

On  looking,  for  instance,  directly  upwards,  the  eyeball 
must  rotate  about  the  horizontal  diameter  of  the  plane.  The 
superior  rectus  cannot  effect  this,  because  its  own  axis  is  inclined 
by  27°  from  it.  It  is,  however,  so  reinforced  by  a  smaller  con- 
traction of  the  inferior  oblique  that  the  resultant  axis  lies  in  the 
plane. 

Elevation  of  the  cornea  is  effected  by  both  of  these  muscles, 
but  it  is  a  "prime"  action  of  the  superior  rectus  and  only  a 
"secondary"  action  of  the  inferior  oblique. 

On  the  other  hand,  intorsion  is  a  "secondary"  action  of  the 
superior  rectus,  and  extorsion  is  the  "prime"  action  of  the  inferior 
oblique.  Since  physiological  elevation  of  the  eye  is  always  quite 
free  from  torsion,  the  two  muscles  must  contract  in  such  proportion 
that  the  intorsion  by  the  one  shall  exactly  counterpoise  the  extor- 
sion by  the  other. 

For  this  to  be  the  case,  the  oblique  must  contract  to  a  much 
less  extent  than  the  rectus  and,  in  reality,  only  about  three-tenths 
of  the  elevation  of  the  eye  is  due  to  the  inferior  oblique,  the  remain- 
ing seven-tenths  being  due  to  the  superior  rectus. 

68 


Associated  Muscles  in  a  Single  Eye 


69 


In  an  exactly  similar  manner  depression  of  the  cornea  is 
effected  by  combined  action  of  the  inferior  rectus  and  the  superior 
oblique. 

In  all  this,  we  are  confining  ourselves  to  motions  which  start  from 
the  primary  position.  If  the  eye  be  adducted  or  abducted  to  start  with,  the 
case  is,  of  course,  different.  Then,  as  Helmholtz  says,  the  resultant  axis  no 
longer  lies  in  the  transverse  plane  of  the 
head,  already  described,  but  in  a  plane 
which  bisects  the  angle  between  it,  and 
the  plane  fixed  in  the  eyeball  which 
originally  coincided  with  the  former  plane 
when  the  eye  was  in  the  primary  position, 
but  which  moves  with  the  eyeball,  so  as 
to  be  ever  perpendicular  to  the  line  of 
fixation. 

This  is  shown  in  Fig.  27,  modified 
from  Helmholtz,*  where  O  B  represents 
the  fixation  line  in  the  primary  position 
of  the  eye. 

The  equatorial  plane  A  A  which 
moves  with  the  eyeball  and  which  passes 
through  the  center  of  motion  perpendicu- 
larly to  the  fixation  line,  takes  the  posi- 
tion CC,  when  the  fixation  line  is  deviated 
from  O  B  to  O  P. 

The  eye  is  now  in  a  secondary  posi- 
tion, and  whatever  motion  it  may  make 

from  this  position  into  any  other,  must  be  effected  by  rotating  about  some 
diameter  of  the  plane  ////which  bisects  the  angle  between  the  planes  A  A 
and  C  C. 

There  are,  of  course,  an  infinite  number  of  diameters  in  this  plane 
(////)  about  which  rotations  are  possible,  so  that  it  may  be  called  "the 
plane  of  the  axes  of  rotation"  for  that  secondary  position  of  the  eye. 

Composition  Of  Rotations.— There  is  a  beautiful  and  well-known 
method  of  representing,  in  linear  measure,  the  amount  of  rotation 
imparted  to  a  rotating  body,  by  simply  measuring  off  along  the 
axis,  from  the  center,  a  distance  proportionate  to  the  rotation 
( ' '  rotation  vector  "  ). 

There  are,  of  course,  two  senses  in  which  a  body  can  rotate 
about  any  one  axis,  and  it  is,  therefore,  needful  to  specify  in  which 
sense  the  rotation  occurs.  This  is  easily  done,  for  since  there  are 
two  directions  in  which  we  can  measure  along  the  axis  from  the 
center,  we  can  choose  one  direction  to  represent  rotation  in  one  sense, 
and  the  other  direction  to  represent  rotation  in  the  opposite  sense. 

* Physiologik.  Optik.,"  p.  624. 


To  show  how  in  an  eye  abducted 
from  B  to  F,  the  axis  of  super- 
duction  is  not  .4  .4  nor  C  C,  but 
in  a  mean  position  (H  H  ). 


70  Tests  and  Studies  of  the  Ocular  Muscles 

By  convention,  we  imagine  ourselves  to  stand  at  the  center  and 
look  along  the  axis  in  that  direction  which  makes  the  motion  appear 
to  us  like  the  hands  of  a  watch  or  the  motion  of  a  right-handed 
screw. 

By  a  single  measured  line,  therefore,  we  can  record  no  fewer 
than  three  quantities  : 

(1)  The  axis  of  rotation,  by  the  direction  of  the  line  ; 

(2)  The  amount  of  the  rotation,  by  the  length  of  the  line  ;  and 

(3)  The  sense  of  the  rotation,  by  the  direction  from  the  center 
in  which  the  line  is  drawn. 

We  may  choose  any  units  we  please.  Suppose,  for  instance, 
we  decide  to  represent  degrees  by  millimeters,  then  10  millimeters 
measured  along  a  direct  line  means  10°  of  rotation  about  that  line 
as  axis,  and  in  the  same  sense  as  that  of  a  screw  being*  screwed 
along  the  direction  of  measurement.  We  have  only  to  seize  the 
line  at  its  origin,  and  screw,  to  understand  the  sense  in  which  the 
rotation  occurs. 

We  may  compound  rotations,  therefore,  or  resolve  them  as  we 
please,  on  the  same  principle  as  the  parallelogram  of  forces. 

Dynamics  Of  the  Eye. — In  theory  the  dynamics  of  the  eye  are 
exceedingly  simple,  since  the  resistances  are  elastic  (and  conform, 
no  doubt,  to  Hooke's  law,  "  Ut  tensio  sic  vis"),  the  forces  are 
tangential,  and  the  lines  of  the  forces  may  with  little  error  be 
reckoned  as  equally  distant  from  the  center  of  motion,  so  that  the 
moments  of  the  forces  are  proportional  to  the  forces  themselves. 
The  ' '  moment "  of  a  force  about  a  point  is  the  importance  of  that 
force  as  regards  balancing  or  producing  rotation  about  that  point. 
The  greater  the  distance  of  the  line  of  force  from  the  point,  the 
greater  is  the  moment  of  the  force. 

Forces  only  Estimated  by  Results. — The  resistances  to  rota- 
tions of  the  eyeball  are  no  doubt  greater  about  some  axes  than 
about  others,  and  since  we  cannot  calculate  this  element,  we  are 
driven  to  study  the  forces  as  if  measured  only  by  the  rotations  they 
produce.  Instead  of  compounding  forces  we  are  obliged  to  com- 
pound rotations,  for  the  forces  are  unknown  quantities  to  us,  while 
the  rotations  can  be  investigated  to  a  high  degree  of  accuracy  by 
the  behavior  of  double  images  and  after-images. 

Fig.  28  illustrates  the  composition  of  rotations  in  a  rotating 
body  whose  center  is  at  o.  The  arrowheads  on  the  lines  o  a,  o  b 
represent  the  directions  in  which  the  lines  are  measured,  and  there- 


Associated  Muscles  in  a  Single  Eye  71 

fore  the  sense  of  the  rotation  which  takes  place  about  each  as  axis 
and  which  is  the  same  as  that  of  an  ordinary  right-handed  screw 
screwed  in  the  direction  of  the  arrow. 

Thus  the  line  o  a  represents  a  rotation  proportionate  to  the 
length  o  a,  and  about  o  a  as  axis,  in  the  sense  of  a  screw  driven 
from  o  to  a.  The  line  o  b  represents  a  smaller  rotation,  since  it  is 
a  shorter  line,  about  o  b  as  axis  and  in  the  same  sense  as  a  screw 
driven  from  o  to  b. 

When  two  forces,  capable  when  acting  singly  of  producing 
these  respective  rotations,  are  impressed  upon  a  body  simulta- 


Fig.  28 

Composition  of  Kotations 

neously,  the  rotation  which  results  is  represented  by  the  diagonal 
o  c  of  the  parallelogram  o  a  c  b  completed  by  drawing  b  c  and  a  c 
parallel  respectively  to  o  a  and  o  b. 

The  resulting  rotation,  therefore,  is  about  the  axis  o  c,  propor- 
tional to  the  length  o  c,  and  in  the  same  sense  as  the  rotation  of  a 
screw  driven  from  o  to  c. 

The  reason  for  this  actual  composition  of  the  rotation  is  as 
follows  :  If  the  body  were  only  subjected  to  one  of  the  rotations 
o  a  or  o  b,  any  point  in  it  would  move  over  a  distance  proportional, 
firstly,  to  the  amount  of  rotation,  and,  secondly,  to  its  distance 
from  the  axis  of  rotation  ;  just  as  the  rim  of  a  wheel  travels  farther 
than  the  hub  during  a  given  rotation,  in  proportion  to  its  distance 
from  the  axle.  When  the  rotations  o  a  and  o  b  take  place  simulta- 
neously, points  which  lie  between  their  axes  would  rise  in  conse- 
quence of  one  rotation  and  sink  in  consequence  of  the  other,  and 
there  is  a  line  of  points  (o  c~)  so  situated  that  the  rising  and  sinking 
exactly  neutralize  each  other.  The  distance  of  each  point  in  this 
line  from  the  two  axes  is  inversely  proportional  to  the  amount  of 


Tests  and  Studies  of  the  Ocular  Muscles 


Fig.  29 

Horizontal  Section  of  a  Right  Eye 


rotation  about  the  axes,  so  that  the  faster  rotation  of  the  body  as  a 
whole  about  one  axis  is  compensated  for  in  the  case  of  the  point 
under  consideration  by  its  greater  distance  from  the  other  axis. 

These  points,  therefore,  all  remain 
stationary  and  form  the  new  axis  of 
rotation.  All  points  which  lie  to 
the  a  side  of  it  are  depressed,  be- 
cause of  their  distance  from  o  b  being 
too  great  to  be  compensated  for  by 
the  greater  rotation  o  a  ;  while  points 
to  the  b  side  of  o  c  are  elevated  for 
the  contrary  reason. 

Now  let  us  apply  these  principles 
to  the  eyeball.  Let  Fig.  29  represent 
a  horizontal  section  of  the  eye,  where 
A  is  the  anterior  pole  of  the  eyeball 

and  P  the  posterior  pole,  so  that  A  P  is  the  optic  axis.  The  line 
D  E  is  the  transverse  axis  ;  /  6"  is  the  axis  of  rotation  for  the 
superior  and  inferior  recti,  and  /'  S'  is  the  axis  of  rotation  for  the 
obliques. 

A  measured  quantity  {Or)  along  the  line  O  S  from  O  as  origin, 
indicates  a  measured  rotation  of  the  globe  in  the  sense  of  a  screw 
proceeding  from  O  to  .5".  This  rotation  elevates  the  cornea  and  is 
such  as  would  be  effected  by  the  superior  rectus  acting,  were  it 
possible,  alone.  Similarly,  any  measured  quantity  (  O  s)  from  O 
towards  /'  specifies  a  proportionate  rotation  by  the  inferior  oblique, 
which  also  elevates  the  cornea,  since  the  sense  of  rotation  is  that 
of  a  screw  passing  from  O  to  /'. 

These  rotations  (Or  and  O  s~),  when  they  occur  simulta- 
neously, are  compounded  into  the  single  rotation  O  E,  which  takes 
place  about  an  axis  in  Listing's  plane. 

Now,  as  in  Fig.  30,  let  us  drop  a  perpendicular  from  A,  the  center  of 
the  cornea,  upon  the  axis  of  the  superior  and  inferior  recti  (/  S).  What 
have  we?  The  vertical  plane  passing  through  this  line  is  the  plane  of 
motion  for  the  center  of  the  cornea  during  isolated  action  of  either  the 
superior  or  inferior  rectus.  The  anterior  pole  of  the  eye  under  these 
conditions  describes  a  circle  in  this  plane,  which  we  might  call  the  corneal 
orbit  for  these  muscles,  since  it  is  the  path  in  which  the  center  of  the 
cornea  travels  under  their  guidance.  (See  Fig.  30). 

A  plane,  therefore,  passing  through  A  perpendicularly  to  the  axis  /  6" 
is  the  plane  of  the  corneal  orbit  during  rotations  about  that  axis ;  and  we 


Associated  l\fusclcs  in  a  Single  Eye 


73 


see  at  once  that  in   whichever  sense  such   rotation   takes   place,  it  must 
necessarily  adduct  the  cornea,  as  well  as  elevate  or  depress  it. 

In  precisely  the  same  way  a  perpendicular  may  be  dropped  from  the 
center  of  the  cornea  (A)  upon  the  axis  of  the  obliques  (/'  Se)  :  the  vertical 
plane  passing  through  this  line  is  the  plane  of  the  corneal  orbit  during 
rotations  about  that  axis,  at  once  indicating  that  abduction  of  the  cornea 
is  a  result  of  such  rotations  whether  they  are 
produced  by  isolated  contraction  or  isolated 
paralysis  of  either  oblique,  if  the  usually  accepted 
view  is  correct  that  the  obliques  have  a  common 
horizontal  axis. 

Resolution  of  Rotations. — Next,  let  us 
see  how  to  resolve  rotations  due  to  indi- 
vidual muscles. 

Take  the  inferior  rectus  as  an  ex- 
ample, and  in  Fig.  29  let  the  distance  O I 
represent  the  maximum  rotation  it  can 
effect.  Drop  perpendiculars  from  /  upon 
the  transverse  axis  D  E  and  the  optic  axis 
A  P;  these  perpendiculars  cut  off  dis- 
tances from  O  along  these  axes  which 
represent  the  component  depression  and 
torsion  respectively. 

Since  thus  O  m  represents  the  depression  of  the  cornea  and 
O  n  its  torsion,  we  see  at  once  that  depression  is  the  prime  action 
of  the  muscle.  The  torsion  occurs  in  the  same  sense  as  in  a  screw 
passing  from  O  to  n,  so  that  it  is  extorsion. 

The  lengths  of  the  lines  O  n  and  O  m  are  easily  found  ;  for 
the  proportion  which  they  each  bear  to  O  I  is  simply  that  of  the 
cosine  of  the  angle  included  between  each  and  O  I,  or,  what  comes 
to  the  same  thing,  of  the  sine,  and  the  cosine  of  /  O  D. 

Suppose,  for  instance,  we  take  the  obliquity  of  the  axis  of  the 
superior  and  inferior  recti  to  be  27°  from  the  transverse  axis,  the 
component  O  m  will  be  .89,  i.  e.,  less  than  nine-tenths;  and 
the  component  O  n  will  be  .45,  i.  e.,  about  nine- twentieths  of  the 
whole  rotation  O  I. 

The  torsion,  therefore,  is  only  about  a  half  .of  the  elevation. 

Co-ordination. — Let  us  now  see  how  much  rotation  the  superior 
oblique  must  effect  in  order  to  be  a  perfect  associate  of  the  inferior 
rectus  (Fig.  29).  Clearly,  if  subduction  is  to  be  unaccompanied 
by  torsion,  the  extorsion  O  n  must  be  counterbalanced  by  an  equal 


Hg.  30 

Horizontal  section  of  a  right  eye. 
The  longer  dotted  liuefrom 
A  indicates  the  vertical 
plane,  to  which  the  motions 
of  the  anterior  pole  of  the 
eye  are  confined  under  the 
guidance  of  the  Sup.  and 
Inf.  Recti.  The  shorter 
dotted  line  indicates  the 
same  for  the  Obliques. 


74  Tests  and  Studies  of  the  Ocular  Muscles 

intorsion  O  ri .  After  marking  off  O  rJ ',  therefore,  equal  to  O  n 
but  in  the  opposite  direction  from  O,  erect  a  perpendicular  at  n'  to 
cut  off  along  O  S'  (the  axis  of  the  obliques)  a  distance  Op,  which 
shows  the  exact  proportion  of  intervention  required  from  the 
superior  oblique  muscle,  its  rotation  being  resolved  into  a  torsional 
component  {O  n')  which  balances  the  torsional  compound  of  the 
rectus  (On),  and  a  subducting  component  equal  to  n' p,  which 
supplements  the  subducting  effect  of  the  rectus.  Indeed,  the 
lengths  O  m  and  n'  p  exactly  represent  the  relative  proportion  of 
pure  subduction  due  respectively  to  the  inferior  rectus  and  superior 
oblique.  The  latter  is  scarcely  more  than  two-fifths  of  the  former.* 

Effect  of  Horizontal  Displacement.— When  the  eye  to  start  with  is 
ab-  or  adducted,  the  proportions  are  different.  We  imagine  the  muscular 
axes  (SI  and  S/  I')  to  remain  fixed  in  space  (though  they  do  not  do  so 
absolutely),  and  the  visual  A  P  and  D  E  to  move  with  the  eye.  In 
abduction,  the  transverse  axis  of  the  eyeball  approaches  the  axis  of  the 
superior  and  inferior  recti. 

With  27°  of  abduction,  therefore,  the  torsional  component  of  the 
superior  and  inferior  recti  ceases,  while  it  would  reach  its  maximum  were 
it  possible  for  the  eye  to  rotate  in  63°.  Conversely,  their  vertical  effect  is 
theoretically  greatest  with  abduction  of  27°,  becoming  nil  with  hypothetical 
adduction  of  63°. 

The  torsional  effect  of  the  obliques  is  greatest  theoretically!  with  abduc- 
tion of  about  39°  and  nil  with  adduction  of  about  51°,  since  in  the  former 
case  the  axis  of  rotation  (S'  I'}  coincides  with  the  optic  axis  (A  P)  and  in 
the  latter  is  perpendicular  to  it.  Exactly  the  opposite  is  true  of  their 
elevating  power,  which  is  nil  with  abduction  of  39°  and  greatest  with 
adduction  of  51°. 

Though  these  calculations  are  at  best  only  approximately 
true,  we  can  by  their  aid  determine  with  more  or  less  approach  to 
truth  the  provinces  of  the  motor  field  over  which  different  muscles 
hold  chief  sway,  or  sway  of  a  special  kind.  A  chart  of  the 
motor  field  on  this  principle  was  attempted  by  Duane. 

The  only  reliable  way  of  constructing  an  exact  chart  of  these 
provinces  is  by  very  careful  examination  and  measurement  of  the 
motor  field  in  cases  of  isolated  paralysis,  since  there  is  some  reason 
to  believe  that  synthetical  calculations  are  only  true  in  a  certain 
measure  owing  to  the  influence  of  Tenon's  capsule,  that  measure 
being  greatest  near  the  primary  position  and  less  with  increasing 
departure  from  it. 

*  n'  p  =  0  m  Tan.  27°,  Tan.  .39°. 

fThe  reason  I  use  the  word  so  freely  in  this  section  is  because  I  suspect  the  muscular  axes 
do  not  remain  quite  so  stationary  as  is  supposed. 


Associated  Muscles  in  a  Single  Eye  75 

The  right-hand  side  of  Fig.  29  (where  we  come  to  deal  with 
the  superior  rectus  and  inferior  oblique)  shows  that  to  resolve  any 
given  superduction  of  the  eye,  such  as  O  E,  we  need  only  com- 
plete the  parallelogram,  of  which  that  line  is  a  diagonal,  by  drawing 
E  r  and  R  s  parallel  respectively  to  the  axis  of  the  recti  and  the 
axis  of  the  obliques.  Then  the  dimensions  O  r  and  O  s  show  the 
component  rotations  effected  by  the  rectus  and  its  associated 
oblique.  They  are  proportional  to  the  sines  of  51°  and  27°  and, 
therefore,  about  17  to  10. 

It  is  true  that  isolated  paralysis  of  the  superior  oblique  is 
common.  The  double  images  therefrom  indicate  sometimes  abduc- 
tion of  the  cornea,  but  far  more  frequently  adduction.  Moreover, 
the  occasional  abduction  is  most  likely  explainable,  on  Mauthner's 
hypothesis,  by  the  liberation  of  a  previously-existing  latent  squint, 
or  tendency  of  the  eyes  to  diverge  (exophoria)  when  not  engaged 
in  single  vision,  from  slack  action  of  the  converging  innervation.* 
If  this  explanation  of  Mauthner's  be  true  and  adduction  be  the 
characteristic  effect  of  this  paralysis  during  the  primary  position  of 
the  sound  eye,  then  the  axis  for  the  muscle  must  be  tipped  up 
above  the  horizontal  plane  at  its  outer  end  and  dip  below  it  at  its 
inner  end,  as  shown  in  Fig.  31. 

Even  then,  adduction  would  only  occur  during  a  moderate  paralytic 
displacement  of  the  eye,  and  would  give  place  to  abduction  if  it  exceeded  a 
certain  amount,  which  it  would  be  quite  easy  to  assign  were  the  exact  tilt  of 
the  axis  known. 

In  fact,  as  soon  as  the  depression  of  the  eye  were  to  become  twice  as 
great  as  the  tilt  of  the  axis,  adduction  would  begin  to  give  place  to  abduc- 
tion, provided  the  center  of  motion  of  the  eye  be  fixed. 

Paralytic  Exophthalmos. — It  should  not  be  forgotten  that  since  the 
four  recti  tend  to  draw  the  eyeball  back  into  the  orbit  (and  balance  thus 
the  tensions  in  the  expansion  from  Tenon's  capsule  to  the  orbit  with  its 
check  ligaments,  and  the  oblique  muscles,  all  of  which  tend  to  draw  the 
eye  forwards,  assisted  by  the  elastic  resistance  of  the  retro-orbital  fat)  it  is 
more  than  likely  that  pronounced  paralysis  of  a  rectus,  when  physiological 
tone  is  lost,  allows  the  center  of  motion  to  advance,  and  thus  the  eyeball  to 
be  translated  forward  as  well  as  rotated.  This,  however,  would  only  intro- 
duce a  source  of  error  into  any  quantitative  calculation,  for  it  would  not 
alter  the  principles  :  the  paralytic  rotation  of  the  globe  would  be  the  same 
in  kind  as  if  no  translation  occurred,  but  less  in  amount. 

It  would,  indeed,  occur  about  an  axis,  exactly  the  same  in  direction  as 
if  there  were  no  translation,  but  which  instead  of  passing  through  the  center 

*  Perfect  orthophoria  (by  whioh  I  mean  orthophoria  maintained  if  one  eye  he  excluded 
for  a  week)  is  not  found  in  one  of  a  thousand :  it  is  this  which  makes  the  horizontal  element 
in  paralysis  so  uncertain. 


75 


Tests  and  Studies  of  the  Ocular  Muscles 


of  motion  would  lie  to  the  opposite  side  of  it  from  the  paralyzed  muscle,  so 
as  no  longer  to  be  a  diameter  of  the  globe.  The  rotation  about  this  new 
eccentric  axis,  however,  would  be  resolvable  into  an  advance  of  the  center 
of  motion  and  a  rotation  about  it,  the  latter  being  the  same  in  kind,  but  less 
in  degree  than  if  there  were  no  translation.  The  greater  the  translation  the 
less  the  rotation.  The  translation  in  itself  is  of  no  clinical  account,  since  it 
does  not  affect  the  diplopia  directly,  but  only  indirectly  by  lessening  the 
amount  of  rotation. 

Model  With  Tilted  Axes.— On  an  india-rubber  ball,  like  Professor 
Landolt's,  I  have  represented,  as  in  Fig.  31,  the  paths  pursued  by  the 
center  of  the  cornea  during  contraction  or  paralysis  of  isolated  muscles 
whose  axes  of  rotation  are  tilted  to  the  horizon.  Since  there  are  four 


TEMPORAL 


lnf.Rect. 


/NA5ALA 
V   SIDE    / 

Sup  RecC. 


Fig.  31 

An  india-rubber  ball,  marked  so  as  to  show  the  Paths  of  the  Cornea  during  Contraction  and 
Paralysis  of  the  Muscles,  if  their  Axes  are  tilted  (the  tilt  being  purposely  exaggerated). 

muscles  concerned,  none  of  which  have  coincident  axes,  there  must  be 
four  corresponding  paths  (or  orbits)  for  the  center  of  the  cornea.  Since 
the  axes  are  not  horizontal,  the  muscular  planes,  to  which  they  are  invariably 
perpendiculars,  cannot  be  vertical  planes,  neither  can  the  planes  of  the 
corneal  orbits,  for  they  are  parallel  to  the  muscular  planes. 

Construction. — We  may,  therefore,  after  deciding  how  much  to  tilt  the 
axes  in  a  model  (e.  g.,  an  india-rubber  ball),  still  follow  Professor  Lan- 
dolt's plan  of  placing  one  leg  of  a  pair  of  compasses  on  the  extremity  of 
each  axis  in  turn,  and  the  other  leg  on  the  center  of  the  cornea  to  describe  a 
circle  with  the  latter.  These  circles  are  the  four  corneal  orbits  for  the 
respective  muscles. 

Differences. — In  a  horizontal-axis  model  there  are  only  two  corneal 
orbits,  each  common  to  a  pair  of  muscles,  and  it  will  be  seen  from  Fig.  23 
that  neither  orbit  transgresses  the  vertical  meridian  :  though  both  touch  it  at 


Associated  Muscles  in  a  Single  Rye  77 

the  corneal  center,  they  keep  strictly  to  their  own  sides,  so  that,  in  the  pri- 
mary position,  adduction  is  the  only  result  demonstrable  by  the  model  of 
either  contraction  or  paralysis  of  the  recti  :  and  abduction  for  the  obliques. 
In  Fig.  31,  however,  each  orbit  crosses  the  vertical  meridian. 

It  may  be  well  to  explain  that  the  model  does  not  represent  the  actual 
globe  of  the  eye,  but  an  infinitely  thin  sphere  immediately  surrounding  it 
and  fixed  in  space,  so  that  the  center  of  the  cornea,  in  its  motions,  describes 
these  paths  upon  it. 

The  vertical  meridian  of  this  sphere  coincides  during  the  primary  posi- 
tion of  the  eye,  with  the  vertical  meridian  of  the  cornea,  but  is  fixed  while 
that  moves.  The  cornea,  when  its  center  is  found  to  the  usual  nasal  side  of 
this  fixed  vertical  meridian,  is  adducted,  when  to  its  temporal  side  abducted. 
From  the  fact  that  each  corneal  orbit  lies  in  part  to  one  side  and  in  part  to 
the  other,  it  is  evident  that  both  adduction  and  abduction  occur  with 
motions  about  each  axis  Each  rectus  (superior  and  inferior)  on  contracting, 
adducts  the  eye,  and  each  oblique  abducts  it. 

The  semi-orbit  A  s  is  that  for  contraction  of  the  superior  rectus,  A  i  for 
the  inferior,  and  both  are  entirely  to  the  nasal  side  of  the  fixed  vertical 
meridian,  showing  these  muscles  to  be  adductors.  Moreover,  since  these 
orbits  form  an  angle  at  the  vertical  meridian  at  the  anterior  pole  instead  of 
touching  it  by  a  continuous  curve,  as  in  Fig.  23,  adduction  is  more  marked 
and  commences  at  once  in  such  a  way  as  not  merely  to  be  an  incident  of  the 
motion  of  the  cornea,  but  to  be  due  in  part  to  true  rotation  of  the  globe 
about  its  vertical  axis,  which  we  have  shown  cannot  occur  rf  the  axes  of 
rotation  for  the  muscular  contractions  are  not  tilted  to  the  horizontal,  since 
horizontal  rotations  cannot  have  vertical  components. 

The  more  tilted  the  axes  are,  the  greater  are  the  vertical  components  of 
their  rotations. 

The  same  may  be  said,  mutatis  mutandis,  of  the  orbits  A  s',  A  i'  for 
the  obliques,  which  show  the  eye  to  be  more  vigorously  abducted  by  con- 
tractions of  these  muscles  than  in  Fig.  23. 

Paralytic  Semi-Orbits. — When  we  come  to  consider  paralytic  rotations, 
the  two  figures  are  in  contrast. 

What  we  may  call  the  "paralytic"  part  of  each  orbit  lies  to  the  oppo- 
site side  of  the  meridian  from  the  "contractile"  part,  for  a  certain  distance, 
and  shows  that  paralysis  of  a  muscle  causes,  at  first,  the  opposite  horizontal 
diplopia  from  its  contraction.  Thus  the  arc  A  s"  is  the  paralytic  arc  for  the 
superior  rectus,  being  continuous  with  its  (already  considered)  contractile 
arc  A  s. 

It  crosses  the  meridian  at  A,  showing  that  the  slightest  paralysis  causes 
abduction  at  once,  which  increases  to  its  maximum  at  b  when  the  depression 
of  the  eye  is  equal  to  the  tilt  of  the  axis  from  the  horizon,  and  then  as  the 
paralytic  rotation  becomes  greater  the  abduction  lessens,  till  the  orbit  again 
crosses  the  meridian  at  c,  thereafter  to  give  place  to  adduction.  This  cross- 
ing of  the  meridian  occurs  when  the  depression  of  the  eye  is  twice  as  great 
as  the  tilt  of  the  axis,  for  c  is  twice  as  distant  from  the  horizontal  meridian 
of  the  fixed  sphere  as  the  extremity  of  the  axis. 


78  Tests  and  Studies  of  the  Ocular  Muscles 

I  have  carefully  said  "as  the  paralytic  rotation  becomes  greater" 
instead  of  saying  "as  the  paralysis  increases,"  because  the  paralytic  rota- 
tion does  not  necessarily  keep  pace  with  the  increase  of  paralysis,  as  we 
have  already  seen,  the  latter  perhaps  expending  its  effect  to  some  extent  on 
translation  of  the  globe  forwards. 

When  this  is  the  case,  what  relation  exists  between  translation  and  rota- 
tion? It  will  not  do  to  compound  them  according  to  ordinary  physical 
composition,  for  the  translation  is  not  something  added  to  the  rotation. 
The  simplest  way  would  probably  be  to  look  upon  the  pathological  yielding 
of  the  muscle  under  remaining  tension  as  represented  by  a  definite  linear 
quantity  of  lengthening.  Without  translation,  this  linear  quantity,  curved 
round  the  surface  of  the  globe  into  a  circular  arc,  measures  the  angle  of 
paralytic  rotation,  being  the  arc  which  subtends  that  angle  at  the  center  of 
motion.  When  translation  occurs,  this  arc  of  rotation  is  shortened  by  a 
linear  quantity  equal  to  the  amount  of  translation. 

Thus,  if  the  lengthening  of  the  muscle  be  3  mm.,  the  angle  of  rotation, 
without  translation,  would  be  an  angle  subtended  by  an  arc  of  3  mm.,  but 
with  i  mm.  of  translation  it  would  be  an  angle  subtended  by  an  arc  of  2  mm. 


CHAPTER   V 


Conjugation  of  the  Two  Eyes 

To  the  best  of  our  knowledge,  every  innervation  of  the  ocular 
muscles  is  conjugate.  It  is  impossible  for  a  nervous  impulse  to 
descend  to  the  ocular  muscles  without  being  equally  divided 
between  the  two  eyes.  In  consequence  of  this  the  two  eyes 
work  together,  to  borrow  Hering's  expression,  as  one  organ. 

It  will  be  seen,  therefore,  that  the  "centrifugal"  impulses  to 
the  eyeballs  answer  completely  to  the  "centripetal"  arrangements 
of  vision.  Homonymous  halves  of  the  two  retinae  convey  impres- 
sions to  the  occipital  lobe  of  the  same  side.  An  object,  for 
instance,  which  lies  in  front  and  to  the  left  throws  its  images  on 
the  right  half  of  each  retina.  If  the  images  fall  on  corresponding 
points  they  are  blended  into  one  in  the  visual  center  into  the  right 
visual  lobe.  In  that  case,  if  attention  be  directed  to  the  object, 
both  eyes  move  equally  and  simultaneously  as  one  organ,  so  as  to 
receive  the  images  of  the  object  on  the  two  maculae  and  inspect  it 
by  direct  vision.  More  often  than  not,  however,  the  object  is  so 
situated  at  first  that  its  images  do  not  fall  on  strictly  corres- 
ponding points.  Then,  simultaneously  with  the  conjugate  lateral 
movement,  an  adjustment  of  the  convergence  takes  place,  in  an 
equally  conjugate  manner,  the  two  movements  being  compounded 
into  one. 

True  Associates. — Every  muscle,  therefore,  has  a  yoke-fellow 
in  the  other  eye.  The  superior  rectus  of  one  eye  is  associated  with 
the  inferior  oblique  of  the  other  ;  and  the  inferior  rectus  of  one  with 
the  superior  oblique  of  the  other. 

Graefe  based  this  view  on  the  secondary  as  well  as  the  prime 
action  of  the  muscles.  For  example,  the  elevating  power  of  the 
right  superior  rectus  is  greatest  on  looking  also  to  the  right  :  and 
that  of  the  left  inferior  oblique  is  likewise  greatest  on  looking  also 
to  the  right.  On  looking  to  the  left,  the  elevating  power  of  both 
decreases,  and  the  torsion  of  both  increases.  Moreover,  their 
torsion  is  in  the  same  sense.  These  muscles,  therefore,  work 
together  more  harmoniously  than  either  could  do  with  any  other 
work-fellow. 

79 


8o  Tests  and  Studies  of  the  Ocular  Muscles 

As  a  matter  of  fact,  however,  in  the  voluntary  motions  of  the 
eyes  this  pair  never  works  alone,  without  the  other  pair  of  associates, 
the  left  superior  rectus  and  the  right  inferior  oblique,  as  we  learn 
from  the  study  of  secondary  torsion  (Chapter  II). 

Graefe  suggested  that  the  best  operative  treatment  for  paralysis 
of  a  muscle  would  be  tenotomy  of  its  associate  in  the  other  eye. 
Thus,  for  a  faulty  right  superior  oblique,  he  would  think  of  tenotomy 
of  the  left  inferior  rectus,  from  the  consideration  that  if  both 
associates  are  weakened  a  stronger  impulse  is  all  that  is  needed 
to  remedy  the  defect  in  both.  There  are  weak  points  in  this 
practice,  excellent  as  the  reasoning  is,  one  being  that  the  propor- 
tion of  elevation  effected  by  the  oblique  is  much  less  than  that 
effected  by  the  rectus,  and  the  other  that  tenotomy  does  not  really 
weaken  a  muscle  much,  but  chiefly  acts  by  altering  the  position 
and  shortening  the  length  of  its  arc  of  contact.  It  does  weaken 
it  a  little,  however,  indirectly,  owing  to  the  lengthening  of  the 
check  ligament,  and  to  that  extent,  it  keeps  company  with  the 
paralysis.  Practically,  the  operation  is  rarely  advisable,  unless  the 
diplopia  is  considerable  while  the  sound  eye  is  in  the  primary 
position. 

Spasm  Of  Single  Muscles. — It  is  rare  to  meet  with  an  unim- 
peachable example  of  those  cases,  described  by  others,  in  which 
there  is  true  idiopathic  spasm  of  an  isolated  muscle.  It  is  possible 
that  ocular  muscles  may  be  subject  to  "cramp,"  like  that  in  the 
calf  of  the  leg,  but  I  have  not  met  with  it.  Were  it  to  occur  in 
any  but  the  internal  rectus,  it  could  easily  be  diagnosed.  Its 
characters  would  be  :  (a)  Sudden,  extreme  and  temporary  devia- 
tion of  one  eye.  (3)  Normal  motions  showing  no  paretic  muscle  in 
the  other  eye.  (f)  Absence  of  marked  heterophoria  in  the  intervals 
between  the  attacks,  as  tested,  by  occlusion  or  the  glass  rod.  This 
is  the  most  important  point  in  the  diagnosis,  for  sudden  deviation 
of  an  eye  may  be  due  to  liberation  of  a  previously-existing  high 
degree  of  latent  squint  and,  unless  paralytic,  is  always  due  to  this. 
Spasm  of  the  internal  rectus  is  often  simulated  by  spasm  of  conver- 
gence, and  since  the  latter  is  always  immensely  more  probable,  it 
should  be  given  the  benefit  of  any  doubt,  (d}  No  other  paretic 
muscle  in  the  affected  eye  discoverable  after  the  attack,  the  nervous 
energy  intended  for  which  may  have  overflowed  into  another,  for 
"secondary  deviation  "  can  be  a  monocular  as  well  as  a  binocular 
affection,  (e)  During  pure  spasm  of  a  single  muscle  there  would 


Conjugation  of  the  Two  Eyes  81 

be  a  temporary  loss  of  concomitancy,  causing  the  squint  to  be 
greater  on  looking  in  some  directions  than  in  others. 

In  chorea  slight  irregular  contractions  of  the  ocular  muscles  are 
said  to  sometimes  take  place,  as  evidenced  by  brief  diplopia. 

In  meningitis  and  other  irritative  affections  of  the  base 
of  the  brain,  irritation  of  the  nerve  trunks  may  cause  spasm 
of  individual  muscles,  though  far  more  frequently  paresis,  or 
both.  It  is  comparable  to  the  rigidity  that  occurs  in  the  limbs 
(Gowers). 

Crampy  or  epileptiform  spasms  of  single  muscles  have  been 
recorded  by  Hock,  Gowers  and  Duane,  in  some  cases  occurring 
when  the  eye  was  moved  into  the  field  of  the  muscle,  in  others 
without  any  exciting  movement  of  the  eye. 

Hysteria  is  said  to  never  affect  single  muscles. 

I  have  dwelt  at  this  length  upon  muscular  spasm,  in  spite  of 
its  extreme  rarity,  because  it  looks  like  an  exception  to  the  rule  of 
"conjugation,"  though  not  really  so,  since  the  pathological  does 
not  disprove  the  physiological. 

Conjugate  Innervations. — The  number  of  conjugate  innerva- 
tions  is,  at  present,  unknown.  Five  have  long  been  recognized  : 
of  which  one  elevates  both  corneae,  another  depresses  them,  a  third 
turns  both  to  the  right,  and  a  fourth  both  to  the  left.  The  fifth  is 
the  converging  innervation. 

Of  the  conjugate  innervations  of  the  eyes  these  five  only  are 
voluntary.  They  are  those  for  the  four  parallel  movements  of 

(1)  Binocular  elevation  ; 

(2)  Binocular  depression  ; 

(3)  Binocular  dextroduction  ; 

(4)  Binocular  laevoduction ;  and  that  for  the  totally  distinct  act  of 

(5)  Convergence. 

Even  parallel  motions  are,  however,  I  find  extremely  difficult 
to  effect  if  the  eyelids  be  kept  closed,  though  they  can  be  brought 
about  by  the  greatest  ease  in  perfect  darkness  if  the  eyelids  be 
opened.  The  act  of  convergence  is  not  so  easy  to  effect  in  dark- 
ness as  parallel  movements,  being  rather  more  dependent  than  they 
on  visual  reflex  government  in  ordinary  life.  It  is  most  easily 
effected  by  thinking  of  a  near  object,  and  it  probably  has  its 
cortical  seat  only  in  the  occipital  lobes  (calcarine  fissure).  Besides 
these  five  innervations,  which  are  more  or  less  under  voluntary 
control,  there  are  two  which  trim  the  torsion  of  the  two  eyes  to 


82  Tests  and  Studies  of  the  Ocular  Muscles 

the  right   and    left   simultaneously  and  which    may,   therefore,  be 
described  as 

(6)  Binocular  dextrotorsion,  and 

(7)  Binocular  Isevotorsion 

These  innervations  are  absolutely  involuntary.  We  know  of 
their  existence  from  physiological  experiments,  clinical  observations 
and  the  phenomena  of  rotational  nystagmus. 

Others  must  have  noticed  what  I  have  sometimes  observed, 
viz. ,  that  after  a  careful  correction  of  astigmatism  the  patient  may 
come  back  needing  a  slight  alteration  of  both  cylinders  by  an  equal 
amount  to  either  the  right  or  left,  showing  that  a  slight  prepon- 
derance of  one  of  the  innervations  for  conjugate  dextroduction  or 
laevoduction  has  occurred  during  the  interval. 

Moreover,  in  paresis  of  an  oblique  muscle,  if  it  happens  to 
belong  to  the  best  eye,  it  is  not  very  rare  to  find  the  tilted  image 
transferred  to  the  unparalyzed  eye,  from  corrective  activity  of  one 
of  these  innervations. 

I  have  recently  seen  a  striking  illustration  of  the  same  kind  of 
transference  in  a  doctor,  whose  left  eye  had  been  blind  for  ten  years 
with  ripe  cataract,  the  vision  of  the  right  eye  being  rather  poor. 
After  extraction  of  the  cataract,  vision  was  wholly  transferred  to 
the  left  eye  and  vertical  objects  appeared  slanting  to  the  left,  prov- 
ing that  the  left  eye  had  become  extorted  during  its  blind  period. 
This  extorsion,  however,  soon  rectified  itself,  but  in  doing  so 
intorted  the  right  eye,  as  shown  by  the  fact  that  on  occluding 
the  left,  objects  viewed  by  the  right  appeared  slanting  to  the  left. 
The  correction,  therefore,  had  been  effected  by  a  conjugate  inner- 
vation,  viz.,  that  of  binocular  dextroduction.  This  correction  did 
not  take  place  once  for  all,  but  ceased  as  soon  as  ordinary  objects 
no  longer  engaged  the  attention,  so  as  to  call  for  it,  as  shown  by 
experiments  with  the  glass  rod. 

It  is  not  scientific  to  speak  of  these  corrections  as  effected  by 
the  obliques.  This  is  well  illustrated  by  rotational  nystagmus,  in 
which  the  two  eyes,  while  experiencing  simultaneous  wheel-move- 
ment, strictly  maintain  their  visual  axes  at  the  same  horizontal 
level,  which  could  not  be  if  the  obliques  only  were  the  active 
agents  :  for  if  they  were,  we  should  find  that  during  double-wheel 
movement,  say,  to  the  right  (binocular  dextrotorsion)  the  superior 
oblique  of  the  left  eye  would  depress  the  left  cornea,  and  the 
inferior  oblique  of  the  right  eye  would  elevate  the  right  cornea. 


Conjugation  of  the  Two  Kycs  83 

The  obliques  could  not  of  themselves  get  rid  of  their  subordinate 
movements. 

Again,  Javal  showed,  by  the  observation  of  astigmatic  correc- 
tion, that  when  we  slope  the  head  towards  either  shoulder  the 
principal  meridians  of  the  two  retinse  no  longer  remain  strictly 
parallel  with  the  median  plane  of  the  head,  but  lag  behind  it  a 
little.;  their  inclination  from  the  true  vertical  becoming  slightly  less 
than  that  of  the  head,  though  they  are  still  parallel  with  each  other, 
an  observation  verified  by  Helmholtz,  with  after-images,  in  a  very 
beautiful  way.  We  have,  therefore,  abundant  evidence  of  the 
existence  of  these  two  innervations. 

It  is  extremely  probable  that  there  are  innervations  for  regulat- 
ing the  parallelism  of  the  vertical  meridians  of  the  retina  with  each 
other,  namely,  one  for 

(8)  Binocular  intorsion,  and  one  for 

(9)  Binocular  extorsion. 

There  is  reason  to  believe  that  if  only  one  of  these  exist,  it  is 
probably  that  for  binocular  intorsion,  and  that  binocular  extorsion 
is  effected  by  its  relaxation  or  inhibition  ;  for  (though  I  speak 
from  general  impressions  only,  and  not  from  statistics)  my 
experience  hitherto  has  appeared  to  show  that  want  of  general 
tone  manifests  itself  rather  by  a  tendency  to  binocular  extorsion 
than  by  intorsion.  Since  the  same  loss  of  tone  occasions  relaxation 
of  the  converging  innervation,  it  may  be  that  binocular  intorsion 
plays  the  same  part  with  respect  to  the  ze'/?<?^/-movement  of  the 
eyes  as  the  converging  innervation  plays  with  reference  to  the 
visual  axes.  If  this  be  so,  intorsion  should  be  designated  as  a  plus 
quantity,  just  as  positive  convergence  is  a  plus  quantity,  and  extor- 
sion would  be  analogous  to  divergence. 

The  weak  point  in  the  demonstration  of  innervations  8  and  9 
is  that  no  case  of  nystagmus  betraying  these  motions  has  yet  been 
recorded,  but  a  great  many  physiological  facts  appear  to  make  at 
least  one  of  them  a  necessity. 

Of  some  other  innervations  we  have  no  positive  proof.  There 
may  very  likely  be  one  for 

(10)  Divergence.      But  until  its  existence  be  demonstrated,  it 
is  safe  to  assume  that  its  place  is  taken  by  relaxation  and  inhibition 
of  the  converging  center. 

Finally,  there  may  be  one  or  two  feeble  innervations  for  pre- 
serving the  two  visual  axes  in  the  "visual  plane"  (z.  e. ,  the  plane 


84  Tests  and  Studies  of  the  Ocular  Muscles 

common  to  the  fixation  point  and  the  centers  of  motion  of  the  two 
eyeballs)  and  by  means  of  which  we  can  overcome  a  weak  vertical 
prism  before  one  eye,  or  in  some  cases  correct  a  high  degree  of 
congenital  hyperphoria.  Lesion  of  the  middle  peduncle  of  the 
cerebellum  has  caused  movement  upwards  of  one  eye  and  down- 
wards of  the  other,  and  this  affords  a  slight  confirmation  of  these 
innervations,  which  would  be 

(n)  One  for  raising  the  right  visual  axis  above  the  left,  and 

(12)  Another  for  raising  the  left  visual  axis  above  the  right. 

The  cortical  seat  of  the  conjugate  innervations  of  the  eyes  has 
been  supposed  to  be  in  several  positions  from  time  to  time,  but  the 
most  reliable  observations  reduce  their  location  to  the  hinder  part  of 
the  mid  frontal  convolution  and  the  median  surface  of  the  occipital 
lobes.  It  is  still  doubtful  whether  the  occipital  centers  avail  for 
visual  reflex  movements  only  or  for  voluntary  also  ;  but  most 
likely  the  voluntary  movements  proceed  from  the  mid  frontal  con- 
volution, stimulation  of  which  causes  movements  of  the  head  and 
eyes  toward  the  opposite  side  of  the  body.  The  inner  surface  of 
each  occipital  lobe  receives  centripital  impulses  from  the  same 
named  halves  of  the  two  retinae  and  since  objects  seen  by  these 
halves  lie  in  the  opposite  side  of  the  field  we  should  expect  stimu- 
lation of,  say,  the  right  cuneus,  to  turn  both  eyes  to  the  left ;  and 
so,  in  fact,  it  has  been  found  to  do  by  experiment.  Schafer  and 
Munk  not  only  found  this,  but  also  that  stimulation  of  the  fore  part 
of  the  visual  center  caused  depression  of  the  eyes  and  of  the 
hinder  part,  elevation.  They  also  found  one  neutral  point,  stimu- 
lation of  which  caused  either  no  motion  or  one  of  convergence 
only. 

The  movements  of  ordinary  life  are  partly  voluntary  and  partly 
reflex.  When  the  voluntary  part  is  impaired  the  reflex  element 
may  preponderate,  as  in  the  following  interesting  patient  of  the 
author's,  where  the  conditions  were  the  exactly  opposite  to  what 
sometimes  obtains  in  paralysis  agitans.  In  that  disease  the  move- 
ments of  the  head  are  slow  compared  with  that  of  the  eyes,  so  that 
when  the  eyes  glance  at  a  new  object,  the  head  slowly  follows  after 
them.  The  reverse  took  place  in  this  case,  the  head  turning  quickly 
to  any  new  object  while  the  eyes  remained  fixed  on  the  last,  and 
only  slowly  regained  the  new  position  of  the  head. 

Accoucheur  reports  that  "  the  mother's  labor  was  very  tedious 
....  had  to  be  completed  with  long  forceps  after  great  difficulty. 


Conjugation  of  the  Tivo  Eyes  85 

the  forceps  having  of  necessity  compressed  the  child's  head  for  a 
very  considerable  time.  The  child' s  breathing  was  established  with 
difficulty  after  its  birth,  but  it  continued  to  breathe  feebly  for  at 
least  a  week,  and  never  cried.  There  was  also  entire  loss  of  the 
power  of  deglutition,  and  the  child  seemed  as  if  it  might  die  at  any 
moment.  It  gradually,  however,  regained  strength  and  the  power 
of  swallowing  to  some  extent.  I  may  mention  that  the  child  was  a 
well-nourished,  healthy-looking  child  at  birth,  and  that  the  marks 
of  the  forceps  showed  that  one  blade  had  been  applied  over  the 
ear  and  the  other  over  the  opposite  cheek  and  temporal  bone. ' ' 

Her  father  reports  that  for  six  months  or  more  she  had  to  be 
carried  about  with  her  head  on  a  pillow,  and  was  only  taught  to 
eat,  i.  e. ,  to  masticate,  by  imitation  ;  she  was  about  three  years 
old  -when  she  began  to  do  this.  Her  orientation,  or  sense  of  the 
direction  in  which  objects  lay,  was  fallacious.  To  correct  this 
defect  in  the  faculty  of  projection,  her  mother  had  to  teach  her  to 
direct  her  hand  to  an  object  to  pick  it  up  by  scattering  pins  on  the 
floor,  and  training  her  to  put  her  hand  in  the  right  direction  for  each. 

The  author  first  saw  her  at  the  age  of  twenty.  She  was  well 
developed,  intelligent,  had  quite  recovered  the  power  of  swallowing, 
but  still  cannot  masticate,  working  the  food  with  her  tongue  instead. 
She  has  spasmodic  contraction  of  the  frontalis  muscles,  alternating 
with  contraction  of  the  orbiculares  palpebrarum.  The  ocular 
movements  are  effected  downwards  without  difficulty,  but  upwards 
they  are  only  about  two-thirds  of  the  normal.  On  telling  her, 
however,  to  look  to  the  right,  she  contorts  her  mouth,  works  her 
frontalis  and  orbicularis  muscles  violently  and,  as  often  as  not, 
begins  by  looking  a  little  to  the  left,  as  if  finding  great  difficulty  in 
directing  the  impulse  into  the  right  channel  ;  at  last  she  succeeds 
in  turning  the  eyes  completely  in  the  direction  asked  for,  though 
not  smoothly.  On  telling  her  to  look  to  the  left,  the  same  kind  of 
thing  happens.  It  is  easier  for  her  to  follow  a  finger  to  the  right  or 
left  than  to  look  from  one  finger  to  another  ;  in  each  case  the  chief 
difficulty  met  with  is  in  crossing  the  middle  line,  a  long  halt  and 
much  working  of  the  eyebrows  being  met  with.  In  a  railway  train 
she  cannot  take  in  what  is  passing.  To  observe  an  object  she  has 
to  keep  moving  her  head  instead  of  her  eyes.  On  asking  her  to 
turn  round  to  look  at  a  person  behind  her,  the  appearance  is  most 
remarkable  :  the  head  goes  round  with  the  body,  but  the  eyes 
remain  as  long  as  they  possibly  can,  till  actually  carried  round  by 


86  Tests  and  Studies  of  the  Ocular  Muscles 

the  canthi  ;  she  is  then  giddy  for  a  moment.  She  never  has 
diplopia  and  never  feels  giddy  except  in  the  act  of  turning  round. 

Here,  then,  is  a  case  in  which  there  is  no  defect  of  the  ocular 
muscles  themselves,  but  extreme  awkwardness  in  effecting  volun- 
tary movements  of  the  eyes  to  the  right  and  left  ;  though  the 
reflex  control  of  this  function  is  not  impaired.  The  contortions  of 
the  face  are  merely  the  overflow  of  misdirected  nervous  energy. 

Conjugate  Paralyses. — The  conjugate  "elevating"  and  "de- 
pressing" innervations  are  sometimes  impaired  by  disease,  so  that 
both  eyes  show  defective  movement  either  upwards  or  downwards, 
without  diplopia  or  any  sign  of  muscular  paresis.  The  exact 
cerebral  localization  of  their  cortical  centers  is  uncertain,  however. 

The  horizontal  conjugate  innervations,  which  impart  lateral 
motions  to  both  eyes,  to  right  and  to  left,  are  also  sometimes 
impaired  by  disease.  One  of  these  innervations  causes  conjugate 
dextroduction,  and  the  other  conjugate  laevoduction.  When  either 
is  defective,  the  field  of  fixation  on  the  same  side  is  proportionately 
restricted,  z.  e. ,  the  eyes  cannot  be  turned  to  that  side,  and  yet 
ihere  is  no  diplopia  or  sign  of  individual  muscles  being  affected. 

The  dextroducting  innervation  acts  equally  upon  the  right 
external  rectus  and  the  left  internal  rectus.  The  laevoducting 
innervation  acts  equally  upon  the  left  external  rectus  and  the  right 
internal  rectus.  It  will  be  seen,  therefore,  that  the  arrangement  is 
very  much  like  that  of  the  reins  of  a  pair  of  horses  (Gowers). 

In  glancing  from  right  to  left,  or  vice  versa,  it  is  evident  that 
there  must  be  a  momentary  median  position  of  the  eyes  in  which 
the  work  is  transferred  from  one  innervation  to  the  other.  Indeed, 
in  some  patients  a  distinct  falter  is  observed  in  this  movement  if 
the  eyes  be  made  to  follow  the  finger  from  right  to  left,  which,  if 
marked,  may  be  regarded  as  a  sign  of  defective  co-ordination. 

Each  of  these  innervations  for  right  and  left  motion  is  supposed 
to  arise  in  the  cortex  of  the  opposite  hemisphere,  and  cross  to  the 
superior  olivary  nucleus  of  the  same  side  ;  thence  to  the  nucleus  of 
the  sixth  nerve  on  the  same  side,  after  which  the  path  divides  into 
two,  one  along  the  sixth  nerve  to  the  external  rectus  of  the  same 
side,  and  the  other  through  the  nucleus  of  the  opposite  third  nerve 
(perhaps  reached  by  the  posterior  longitudinal  bundle)  to  the 
opposite  internal  rectus.  Any  interference  with  this  innervation, 
therefore,  must  be  due  to  some  defect  higher  up  than  the  division 
of  its  path  into  two  :  it  must  be  either  in  or  above  the  sixth  nucleus. 


Conjugation  of  the  Two  Eyes 


Convergence. — Were  these  four  innervations  the  only  ones  to 
control  the  eyes,  their  axes  of  fixation  would  be  ever  parallel.  A 
fifth  is  necessary  to  converge  the  eyes  upon  near  objects,  and  it  is 
found  to  have  quite  an  individuality  of  its  own. 

It  also  affects  both  eyes  equally,  and  though  its  cortico-nuclear 
path  is  not  well  ascertained,  its  nucleo-muscular  path  is  well  known, 
since  it  certainly  passes  through  the  third  nerve  nuclei  and  divides 
into  two  paths,  each  of  which  courses  down  the  branch  of  the  third 
nerve,  which  supplies  the  internal  rectus  of  its  own  side. 

Entire  absence  of  the  converging  innervation  is  met  with, 
though  very  rarely  ;  considerable  defects  are  not  so  very  uncom- 
mon, and  trifling  ones  are  exceedingly  common  ;  in  all  of  which 
the  innervations  previously  described  may  be  perfect. 

Monocular  Motion. — Though  all  the  innervations  of  the  eyes 
are  equally  divided  between  the  two  members,  it  does  not  follow 
that  motion  of  one  eye  necessitates  motion  of  the  other. 

Fig.  32.  for  instance,  which  is  borrowed,  with  a  slight  modifi- 
cation, from  Hering,  shows  how 
one  may  remain  quite  stationary 
while  the  other  moves,  without  in 
the  least  impeaching  the  rule  of 
equally  divided  innervations. 

The  lines  y  and  z  represent 
parallel  visual  axes,  while  the  eyes 
are  looking  straight  forwards  at 
some  very  distant  object.  Let  the 
attention  be  now  diverted  to  a  near 
object  (V)  lying  in  the  line  of  the 
right  visual  axis  z,  so  that  the  right 
eye  has  no  motion  to  make  in  look- 
ing at  it,  but  the  left  eye  has  to 
sweep  through  the  angle  y  A  c. 
Hering  has  shown  that  half  the 
motion  of  the  left  eye  (y  A  o)  is 
due  to  the  converging  innervation, 
and  the  remaining  half  (o  A  c}  to 
the  innervation  which  turns  both 

eyes  to  the  right,  and  that  while  the  two  innervations  conspire  in 
the  case  of  the  left  eye,  they  exactly  counteract  each  other  in  that  of 
the  right.  The  left  internal  rectus  is  stimulated  by  both,  and  the 


Fig.  32 

Modified  from  Hering,  to  illustrate 
Conjugation.  In  looking  at  c  the 
eyes  converge  as  if  for  o,  and  are  as 
if  deflected  from  o  to  c. 


88 


Tests  and  Studies  of  the  Ocu!ar  Muscles 


left  external  rectus  by  neither  ;  whereas,  the  right  internal  rectus  is 
stimulated  by  one,  and  the  right  external  also  by  one.  Convergence, 
therefore,  brings  both  eyes  as  if  to  o,  through  the  equal  angles  y  A  o 
and  z  B  o,  and  dextroduction  moves  them  as  if  from  o  to  c,  through 
the  equal  angles  o  A  c  and  o  B  c  ;  not,  of  course,  that  they  act  in 
succession  but  simultaneously,  so  that  every  increment  of  converging 
force  is  instantaneously  prevented  from  moving  the  right  eye  by 
an  equal  increment  of  dextroducting  force.  The  mechanism  is 
wonderfully  perfect. 

It  may  be  noticed  that  in  the  diagram  (Fig.  32),  instead  of 
following  Hering  by  making  o  c  a  horizontal  straight  line,  I 
have  made  the  point  o  a  little  farther  from  the  eyes  than  c ;  for 
were  it  not  so,  the  angles  would  not  be  equal.  They  are  only 
equal  when  they  lie  in  the  same  circular  arc  as  the  centers  of  motion 
of  the  two  eyes. 

Fig.  33  is  to  show  this,  where  the  circle  A  B  O  c  repre- 
sents what  I  have  called 
an  "  isogonal  line,"* 
or  line  of  equal  conver- 
gence. All  points  in  the 
periphery  of  this  circle, 
when  made  objects  of 
fixation,  require  an  equal 
amount  of  convergence. 
That  explains  the  dotted 
line  between  c  and  o  in 
Fig.  32  being  in  the  form 
of  a  curve,  instead  of  a 
straight  line. 

In  Fig.  33  it  is  not 
only  true  that  the  angles 
of  convergence  are  equal, 
but  in  glancing  from  any 
one  point  in  the  circle  to 
any  other,  both  visual  axes 
traverse  equal  angles :  thus 

the  angles  O  B  c  and  O  A  c  are  equal.  It  is  the  curve,  there- 
fore, both  of  uniform  convergences  and  of  equal  lateral  ductions 
for  the  two  eyes. 

*"  Journal  of  Anat.  and  Phys.,"  vol.  xxi.,  p.  581. 


Fig.  33 

(Syrae  Fellowship  Essay,  1884. )  To  illustrate  the  rela- 
tion between  convergence  and  accommodation  in 
lateral  fixation.  Looking  at  c  requires  conver- 
gence as  if  for  O,  and  accommodation  as  if  for  r. 


Co)ijugation  of  the  Two  Eyes  89 

It  must  not  be  thought  that  the  rules  of  conjugation,  so  well 
investigated  by  Hering,  exist  merely  in  theory  ;  they  are  exceed- 
ingly well  proved. 

Confirmation  of  the  Rules  of  Conjugation.— The  reader  can, 
if  he  please,  confirm  them  for  himself,  by  fastening  a  square  black 
velvet  to  a  thin  board,  and  fixing  a  tiny  piece  of  white  paper,  say, 
2  mm.  square,  at  its  center.  Holding  the  board  about  8  inches 
before  the  face,  look  at  the  tiny  piece  of  paper,  and  suddenly  cover 
the  right  eye  with  a  visiting  card. 

To  a  keen  observer  the  white  spot  will  now,  in  most  cases, 
appear  to  move  slowly  to  the  right.*  Now  here  is  a  remarkable 
phenomenon  :  The  point  of  view  seems  to  move  when  not  only  is  it 
really  stationary,  but  the  eye  which  looks  at  it  and  the  image  it 
throws  upon  the  retina  are  stationary  also. 

That  the  covering  of  the  right  eye  does  not  make  the  left  eye 
move  may  be  proved  by  placing  a  circular  piece  of  paper,  half  an 
inch  broad,  on  the  velvet  screen  just  where  it  is  lost  to  view  in  the 
blind  spot  of  the  left  eye,  and  such  that  any  motion  of  the  eye 
would  make  the  paper  spring  at  once  into  view.  It  will  be  found 
that  covering  the  right  eye  does  not  make  it  spring  into  view  :  the 
left  eye,  therefore,  does  not  move. 

What,  then,  is  it  that  makes  the  white  spot  fixed  by  it  seem 
to  move  ? 

Simply  this,  that  when  the  right  eye  is  covered,  the  necessity 
for  strict  convergence  ceases,  and  the  converging  innervation 
relaxes  a  little.  Were  this  all,  both  eyes  would  diverge  a  little, 
but  that  would  make  the  left  eye  deviate  as  well  as  the  right  :  to 
prevent  the  left  eye  from  moving,  every  relaxation  of  convergence 
is  simultaneously  compensated  for  by  a  corresponding  increment  of 
nervous  energy  from  that  innervation  which  turns  both  eyes  to  the 
right.  This,  while  it  just  counteracts  the  divergence  of  the  left 
eye,  increases  the  divergence  of  the  right. 

Now,  in  estimating  the  position  of  the  white  spot  on  the 
screen,  the  mind  pays  no  attention  whatever  to  the  behavior  of  the 
converging  innervation  (unless  to  make  the  white  spot  appear  to 
recede  to  a  distance,  as  Percival  experiences  when  he  tries  the 
experiment),  but  is  keenly  alive  to  the  slightest  output  of  energy 
by  the  other  innervation,  and  judges  the  gradual  evolution  of  its 


*This  experiment  is  best  made,  and  indeed  was  originally  made,  with  the  visual  camera 
(Chapter  XIV). 


90  Tests  and  Studies  of  the  Ocular  Muscles 

energy  to  be  due  to  motion  of  the  white  spot,  instead  of  attributing 
it  to  its  true  cause. 

This  experiment  alone  would  suffice  to  prove  that  the  muscular 
sense,  in  the  case  of  the  ocular  muscles,  is  not  peripheral  but 
central,  since  it  is  the  kind  and  amount  of  central  innervation  that 
determines  the  judgment  of  localization  in  space,  not  the  muscular 
tension  excited  by  it. 

Now,  to  continue  our  experiment  :  After  holding  the  card 
before  the  right  eye  for  a  full  minute,  suddenly  remove  it  :  two 
white  points  appear  which  run  into  one,  and  it  will  be  seen  that 
they  both  move  at  equal  rates  to  meet  each  other,  so  that  the 
previously-slow  movement  of  the  first  one  is  rapidly  retraced. 

Why  is  this  ? 

It  is  because  the  sudden  apparition  of  double  images  at  once 
awakens  the  desire  to  unite  them  and  quickens  the  converging 
innervation,  which  acts  on  both  eyes  alike,  to  do  so.  The  left  eye, 
however,  all  the  time,  does  not  stir  (as  we  can  prove,  if  we  wish, 
by  the  blind-spot  method),  for  as  quickly  as  its  internal  rectus 
experiences  the  converging  stimulus,  it  loses  the  previous  dextro- 
ducting  stimulus. 

It  is  the  cessation  of  the  dextroducting  stimulus  which  in  this 
experiment  makes  the  white  point  appear  to  move  to  the  left. 

On  the  other  hand,  half  the  corrective  movement  of  the  right 
eye  is  due  to  converging  impulse,  and  the  other  half  to  the  cessa- 
tion of  dextroduction. 

The  practical  perfection  of  this  mechanism  is  most  important 
in  the  little  details  of  life. 

Mental  Appreciation  of  Parallel  Innervation. — We  have  proved 
that  the  mind  takes  the  most  careful  cognizance  of  the  least  output 
of  energy  by  the  innervations  which  cause  parallel  motions  of  the 
eyes.  So  much  is  this  the  case  that  artists  are  said  to  be  able  to  judge 
more  correctly  the  lateral  distance  between  two  objects  by  glancing 
rapidly  from  one  to  the  other  than  by  any  other  visual  method. 

Mental  Appreciation  of  Converging:  Innervation. — What  now 
about  the  converging  innervation  ?  Does  the  mind  take  no  cogni- 
zance of  it?  Yes,  but  in  a  totally  different  way.  It  speaks  to  the 
mind  only  of  the  distance  of  objects,  not  in  the  least  of  the  direction 
in  which  they  lie. 

Converging  impulses  affect  both  eyes  equally,  and  since  in 
looking  at  near  objects,  the  eyes  have  to  converge  more  strongly 


Conjugation  of  the  Tico  Aj'cs  91 

than  for  remoter  ones,  the  sense  of  nearness  is,  c&teris  paribus, 
proportionate  to  the  effort  put  forth 

The  mental  estimate  of  convergence,  however,  is  not  so 
minutely  exact  as  for  parallel  motions  of  the  eyes.  The  reason 
of  this  may  be  that  we  possess  no  other  means  of  telling  the 
direction  of  an  object  than  by  the  parallel  innervations  of  the  eyes, 
but  their  distance  is  known  to  us  by  their  apparent  size,  by  atmos- 
pheric effects,  by  perspective,  by  stereoscopic  phenomena,  and  by 
the  effort  of  accommodation  required  ;  and  (we  must  also  add)  our 
knowledge  of  the  relative  position  of  objects  or  their  surroundings 
from  experience. 

Convergence  and  Accommodation. — This  leads  us  naturally  to 
treat  (though  briefly)  of  the  association  between  convergence  and 
accommodation.  The  accommodating  innervation  affects  the  two 
ciliary  muscles  in  just  as  conjugate  a  manner  as  the  converging 
innervation  affects  the  two  recti.  This  is  believed  to  be  the  case 
even  when  the  two  eyes  are  congenitally  of  different  refraction  ;  so 
we  may  conclude  that  the  innervations  of  the  eyes  are  not  entirely 
disposed  by  habit. 

When  we  look  at  a  very  distant  object,  convergence  and 
accommodation  are  both  nil,  and  they  increase  part  passu  as  the 
object  approaches. 

This  intimate  correlation  between  the  two  actions  is  in  such 
perpetual  exercise  during  the  waking  hours  of  life  that  we  might 
naturally  wonder  at  first  thoughts  whether  one  single  innervation 
would  not  have  served  the  purpose  of  two. 

I  will  content  myself  here  with  giving  one  reason  why  this 
could  not  be,  on  geometrical  grounds,  unless  all  vision  were 
directed  to  objects  in  the  median  plane.  Whenever  the  eyes  are 
turned  to  the  right  hand  or  the  left,  a  differing  proportion  between 
convergence  and  accommodation  is  necessitated  :  for  slight  lateral 
motions  of  the  eyes,  accommodation  needs  to  be  relatively  increased, 
but  as  soon  as  the  motion  exceeds  a  certain  limit,  the  necessity  is 
reversed,  and  the  greatest  demand  is  for  convergence. 

Let  us  look  at  each  in  turn,  and  consider  : 

(a)  Accommodation. — Apart  from  any  connection  with  con- 
vergence, disproportion  between  accommodative  requirements  in 
the  two  eyes  respectively  is  brought  about  by  the  slightest  deviation 
of  the  point  of  fixation  from  the  median  plane,  except  along  one 
curve  only. 


92 


Tests  and  Studies  of  the  Ocular  Muscles 


Shows  the  different  amount  of  Accommodation  called 
for  in  the  two  eyes  on  lateral  fixation 


Fig.  34  illustrates  this  when  any  flat  object  is  looked  at,  as  in 
reading  a  book.  The  prolongations  of  the  visual  lines  on  the 
distal  side  of  the  line  A  B  represent  the  disproportion. 

Thus,  when  both  eyes 
are  looking  at  n,  the  ob- 
ject is  nearer  to  the  right 
eye  than  to  the  left  by  the 
distance  /  u,  and  so  on. 
Every  departure  of  the 
point  of  fixation  from  the 
middle  line  lessens  the 
required  accommodation 
in  the  opposite  eye,  while 
at  first  it  increases  that  of 

F1e-  34  the  eye  of  the  same  side 

till  the  fixation  point  has 
traversed   a    distance  o  p 

equal  to  half  the  interocular  distance,  after  which  it  falls  through 
a  similar  interval  p  q  to  the  original  amount,  and  then  con- 
tinuously diminishes. 

But  the  centers  for  accommodation  are  so  intimately  connected 
that  one  eye  does  not  normally  accommodate  more  than  the  other. 
When  variations,  therefore,  exist  either  in  the  refractive  power  or 
requirements  of  the  two  eyes,  "  that  eye  has  the  bright  image  which 
attains  it  most  easily  at  the  expense  of  the  other"  (Donders). 
They  do  not  split  the  difference  ;  if  they  did  so  there  would 
be  diffusion  circles  in 
each  eye. 

Since  accommoda- 
tion with  normal  refrac- 
tion implies  positive 
effort,  that  eye  which  is 
farthest  from  the  object 
and  can  see  it  with  least  Fig.  35 

effort,    determines     the       Line  of  oqnai  Accom- 

niodatiou 

accommodation  for  both. 

Fig.  35,  therefore,  represents  "the  line  of  equal  accommo- 
dation" for  near  vision,  made  up  of  two  curves,  in  whatever 
point  of  which  the  object  is  placed  accommodation  remains  the 


G— -O 


Q— --0 


Fig.  36 

Line  of  equal  Conver- 
gence 


Conjugation  of  the  Two  Eyes  93 

same.  It  is  composed  of  two  arcs  of  equal  radius,  described  from 
the  centers  of  their  opposite  eyes.* 

(b~)  Convergence. — Fig.  34  shows  that  convergence,  as  well 
as  accommodation,  diminishes  with  oblique  vision  ;  but  that  they 
do  not  diminish  equally  is  made  clear  by  the  fact  that  the  line  of 
equal  convergence  (Fig.  36)  is  not  of  the  same  shape  as  that  of 
equal  accommodation  (Fig.  35). 

The  equal-convergence  curve  in  Fig.  36  is  part  of  a  circle  which 
passes  through  the  centers  of  motion  of  the  two  eyes  (not  like  the 
horopter  through  the  nodal  points)  and  possesses  these  properties  : 

(1)  The  angle  of  convergence  is  the  same  whatever  point  in  it 
is  made  the  point  of  binocular  fixation. 

(2)  In  glancing  from  any  one  point  in  it  to  any  other,  both 
visual  axes  traverse  equal  angles  ;  thus  in  Fig.  33  the  angles  O  B  c 
and  O  A  c  are  equal  ;  while,  in  contrast  to  this,  Fig.  34  shows  that 
in   glancing  from  o  to  p  or  o  to  q  the  left  axis  passes  through  a 
greater  angle  than  the  right  ;  as  it  does,  indeed,  whatever  point  is 
looked  at  to  the  left  in  the  straight  line  A  B. 

(3)  The    line  which  bisects  the  angle  of  convergence  is  the 
one  to  which  (hypothetically)  objects  upon  the  maculae  should  be 
mentally  referred.      Whether,  in  fact,  they  are  so  physiologically, 
is  another  question,  as  we  shall  see. 

The  line  is  obtained  by  uniting  the  point  of  binocular  fixation 
(c,  in  Fig.  33)  to  the  posterior  point  of  the  circle  (^).  The  posi- 
tion of  this  posterior  point  shifts,  slightly,  of  course,  with  every 
variation  in  the  size  of  the  circle. 

The  line  c  b  itself  is  inclined  to  the  median  plane  by  an  angle 
which  measures  the  obliquity  of  vision,  since  it  is  equal  to  the 
angle  which  each  visual  axis  has  traversed  in  looking  from  the 
anterior  point  of  the  circle  (  0}  to  any  other  point  in  it  (r). 

Curves  Applied  to  Each  Other. — In  Fig.  33  the  dotted  arcs 
represent  the  line  of  equal  accommodation,  so  applied  to  that  of 
equal  convergence  as  to  illustrate  the  fact  already  mentioned,  that 
within  a  certain  degree  of  obliquity  of  vision  the  proportion  of 
convergence  to  accommodation  is  greater  than  in  the  median  plane, 
while  for  greater  obliquity  the  proportion  is  less. 

*In  hypermetropia  the  line  is  similar;  the  reasons  for  it  being  so  are  intensified,  since 
accommodation  is  a  greater  effort  ;  but  in  myopia  accommodation  is  negative  outside  the  line 
which  limits  the  far  point,  and  which  is  made  by  drawing  each  arc  from  the  center  of  the  eye 
of  the  same  siile.  Since  in  those  cases  relaxation  is  often  attended  with  more  effort  than 
accommodation  (owing  to  spasm  of  the  eil'ary  muscle),  the  "line  of  equal  accommodation" 
would  he  of  the  same  shape  as  the  "  fur-point  line  "  for  some  distance  within  it. 


94  Tests  and  Studies  of  the  Ocular  Muscles 

At  the  points  d  d,  where  the  two  lines  intersect,  the  propor- 
tion between  convergence  and  accommodation  is  the  same  as  at  O  ; 
within  these  points  convergence  must  be  relatively  increased  ;  out- 
side of  them  relatively  lessened.  The  distance  of  each  d  from  O  is 
always  exactly  equal  to  the  inter-central  distance  of  the  observer. 

The  same  figure  shows  how,  in  looking  obliquely  at  any  point, 
the  convergence  and  accommodation  required  may  be  compared 
with  that  in  the  median  plane.  All  that  is  necessary  for  any  given 
point  (c)  is  to  describe  a  circle  through  it  and  through  the  center 
of  each  eye,  as  in  the  figure,  and  from  the  center  of  the  farthest 
eye  (B}  to  draw  the  arc  c  x  from  c  to  the  median  line.  The  circle 
is  the  line  of  equal  convergence  for  that  point,  and  the  arc  is  part 
of  the  line  of  equal  accommodation  for  the  same. 

In  binocular  vision,  therefore,  of  the  point  c,  convergence  must 
occur  as  if  for  O,  and  accommodation  as  if  for  x,  while  both  eyes  are 
deviated  to  the  right  through  the  equal  angles  O  B  c,  o  A  c. 

The  point  b  gives  the  base  of  the  line  which  bisects  the  angle 
of  convergence,  and  which  is  made  by  joining  b  c,  while  the  inclina- 
tion of  this  line  to  the  median  plane  expresses  the  angular  parallel 
movement  of  the  two  eyes.* 

The  lines  of  equal  convergence  and  accommodation,  if  rotated 
round  the  interocular  line  as  an  axis,  would  describe  surfaces  of 
equal  convergence  and  accommodation  respectively,  f 

Impulse  and  Work. — This  angle,  therefore  (O£c\  represents 
the  dextroducting  or  laevoducting  work  to  be  done  in  looking  at 
any  point  (V).  I  do  not  say  the  impulse,  but  work,  for  effort  is 
often  disproportionate  to  work,  owing  to  greater  resistance  or  other 
disadvantages. 

A  certain  evolution  of  nervous  energy  from  the  converging  center 
produces  a  definite  angular  deflection  inwards  of  both  visual  axes,  and 
similarly  an  impulse  of  a  lateral  center  produces  a  definite  deviation 
of  both  visual  axes  to  the  right  or  left.  The  nervous  impulses  per- 
form angular  work,  if  I  may  so  say,  as  far  as  vision  is  concerned  and, 
therefore,  we  may  assume  that  it  is  by  angles  that  the  mind  judges 
of  the  work  done  in  estimating  the  projection  of  the  field  of  vision  ; 

*For  any  point  of  fixation  (c)  the  isogonal  circle  may  be  found  by  drawing  from  A  and  B 
straight  lines  at  right  angles  to  c  A,  c  B  respectively.  From  the  point  where  they  meet  draw  a. 
straight  line  to  c,  the  bisection  of  which  gives  the  center  of  the  required  circle.  Or  7-,'i/r//«/.  iv,  5. 

t  (1)  To  find  the  surface  of  equal  convergence — the  radius  of  the  required  circle  is  found 
by  dividing  half  the  intercentral  distance  by  the  sine  of  the  angle.  (It  may  be  found 
geometrically  by  Euclid,  iii,  33.)  (2)  To  find  the  angle  of  convergence  when  an  object  is 
viewed  in  the  median  plane  at  a  given  distance  from  the  eyes — half  the  interocular  distance, 
divided  by  the  distance  of  the  object  from  the  eyes,  gives  the  sine  of  half  the  angle.  Or  half  the 
interocular  distance  divided  into  the  distance  from  the  eyes  gives  the  uuniher  of  meter-angles. 


Conjugation  of  the  Two  Eyes  95 

but  since  the  judgment  is  based  solely  upon  the  impulse  put  forth, 
any  discrepancy  between  it  and  work  would  show  itself  in  angular 
misjudgment,  unless  by  habit  the  mind  had  come  to  associate  a  cer- 
tain degree  of  impulse  with  the  work  it  usually  performs,  instead  of 
with  the  work  it  should  perform  compared  with  a  smaller  effort. 

Such  allowance  is  no  doubt  made,  in  whole  or  in  part,  except 
for  unusual  obliquities. 

Were  "impulse"  and  "work"  exactly  proportionate  in  the 
parallel  motions  of  the  eyes,  all  objects  seen  by  the  fovea  of  one  eye 
or  both,  however  obliquely,  would  be  referred  to  the  line  which 
bisects  the  angle  of  convergence,  since  its  inclination  to  the  median 
plane  would  exactly  express  the  angular  impression  in  the  mind 
produced  by  the  lateral  effort. 

Experiment  in  Impulse  and  Work. — I  find,  however,  that  if  a 
large  piece  of  card  be  held  very  much  to  one  side,  a  few  inches  in 
front  and  to  the  outer  side  of  one  eye,  with  a  mark  upon  its  upper 
border,  looked  at  with  both  eyes,  a  finger  passed  up  behind  it 
generally  at  first  misses  the  mark  to  the  outside  by  nearly  an  inch, 
showing  that  the  lateral  impulse  is  relatively  so  far  greater  than  the 
work  it  accomplishes  that  the  mind  estimates  as  if  more  angular 
work  were  done,  and  mentally  displaces  the  object  from  the  median 
plane  by  an  angle  greater  than  that  of  the  line  which  bisects  the 
angle  of  convergence  and  which  only  measures  the  work  actually 
accomplished,  not  the  effort  put  forth  to  accomplish  it. 

Why  Convergence  and  Accommodation  are  Not  Inflexibly  One.— 
Were  the  relation  between  convergence  and  accommodation  inflexibly 
complete  for  objects  in  the  middle  line,  there  would  be  diplopia  for 
any  object  out  of  the  middle  line,  except,  perhaps,  at  one  point  on 
each  side,  within  which  diplopia  would  be  heteronymous  from  rela- 
tive divergence,  and  without  which  it  would  be  homonymous  from 
relative  convergence.  The  nervous  ties,  therefore,  though  strong 
enough  to  relieve  fusion-effort,  are  not  so  strong  but  what  they  can 
be  overcome  to  meet  such  requirements. 

Second  Reason. — Apart  from  these  geometrical  considerations, 
it  is  found  that  the  more  the  eyes  look  to  either  side,  the  greater 
becomes  the  practical  difficulty  of  converging  the  eyes,  so  that  the 
excess  of  converging  effort  required  over  accommodating  effort 
increases  in  proportion  to  the  amount  of  lateral  deflection. 

Exophoria  in  Near  Oblique  Vision.— Thus  Dr.  Joseph  Bolton,  of 
Nottingham,  found,  on  experimenting  with  a  modification  of  the  visual 


96  Tests  and  Studies  of  the  Ocular  Miiscles 

camera,  adapted  for  the  purpose,  the  following  deficiencies  in  conver- 
gence, with  accommodation  for  10  inches,  his  eyes  being  dislocated  : 


FOR  AN  OBJECT  10  INCHES  DISTANT 

EXOPHORIA 

On  looking  straight  forwards  

—  6° 

Looking  10°  to  the  right     

—  7°  10' 

20°        "               "           

—  8°  54' 

-     0            ««                      .< 

—  10°  45' 

"         «°      "           " 

—  12°  36' 

The  last  figures  show  the  visual  axes  to  be  actually  divergent, 
so  that  if  prolonged  backwards  they  would  meet  at  a  point  behind 
the  head.  It  is  indeed  just  as  much  as  we  can  do  to  overcome  this 
tendency  to  diplopia,  in  the  lateral  limits  of  the  field  of  fixation,  as 
the  following  simple  experiment  will  show  : 

Experiment  in  Peripheral  Diplopia.— Hold  an  ordinary  lead 

pencil  about  three  inches  to  the  outer  side  of  the  right  eye  and, 
with  the  right  eye  closed,  advance  it  just  sufficiently  to  let  it  be 
visible  to  the  left  eye  across  the  root  of  the  nose.  Now,  open  the 
right  eye,  when  double  images  at  once  appear,  of  which  the  left- 
hand  one  belongs  to  the  right  eye  and  the  right-hand  one  to  the 
left  eye.  It  will  be  found  that  it  needs  an  appreciable  effort  to 
unite  them.  The  fact  that  the  images  are  "crossed"  shows  that 
the  eyes  are  not  sufficiently  converged. 

Relative  Range  of  Accommodation.— Donders  showed  that  to 
each  fixed  quantity  of  convergence  there  is  attached  (for  the  same 
person)  a  definite  range  of  relative  accommodation  ;  that  is,  there 
are  well-defined  limits  within  which  accommodation  can  be  made 
to  exceed  convergence  or  come  short  of  it  by  the  forced  employ- 
ment of  concave  and  convex  lenses. 

Relative  Range  of  Convergence. — For  each  fixed  quantity  of 
accommodation  there  are  definite  limits  (in  the  same  person) 
within  which  convergence  can  be  lessened  or  increased  by  prisms  ; 
lessened  by  prisms  with  their  apices  outwards,  and  increased  with 
them  inwards.  Since  prisms  are  not  quite  so  easy  to  work  with  as 
lenses,  this  subject  has  not  yet  been  so  satisfactorily  worked  out  as 


Conjugation  of  the  Two  Eyes  97 

the  relative  range  of  accommodation.     The  two    ranges  with  the 
same  person  are,  of  course,  correlated. 

Latent  Deviations  (Heterophoria). — In  our  experiment  with 
the  black  velvet  we  have  had  occasion  to  notice  that  an  eye 
occluded  in  near  vision,  deviates  outwards  under  the  screen. 
When  this  was  observed  only  in  its  more  extreme  manifestations 
and  was  considered  a  pathological  occurrence,  it  went  by  the  name 
of  "insufficiency  of  the  internal  recti,"  till  I  was  able  to  show,  in 
1882,  by  the  visual  camera  (Chapter  XIV)  it  is  a  physiological 
occurrence  in  nearly  every  one,  to  an  amount  averaging  nearly  four 
degrees  with  accommodation  for  ten  inches,  and  that  as  the  object 
of  vision  is  made  to  recede,  it  gradually  lessens  to  practical  zero  in 
distant  vision.  It  is  not  due  to  any  fault  of  the  internal  recti,  but 
simply  to  a  flagging  of  the  conjugate  innervation  of  convergence 
when  there  is  no  work  for  it  to  do.  Many  pathological  deviations 
occur,  which  are  treated  of  in  Chapter  XII. 


CHAPTER   VI 


Fixation,  Projection  and  Binocular  Vision 

The  object  of  ocular  movements  is  to  bring  the  best  point  of 
the  retina  to  bear  upon  objects  looked  at,  and  thus  obtain  the 
keenest  possible  vision  of  whatever  point  engages  the  will  or  the 
attention  at  the  moment. 

When  our  attention  is  directed  to  any  point,  the  eyes  almost 
simultaneously  follow,  and  by  making  them  glance  from  point  to 
point,  so  as  to  see  a  series  of  pictures,  we  gain  a  true  conception  of 
the  shape,  size,  solidity  and  position  of  objects. 

At  other  times,  the  eyes  seem  to  wander  of  themselves,  almost 
involuntarily,  till  attention  is  suddenly  awakened  by  what  they  see. 

Point  Of  Fixation — The  point  which  for  the  moment  engages 
an  eye  is  called  "the  point  of  fixation,"  and  by  a  curious  reversal 
of  terms  the  eye  is  said  in  ophthalmological  language  to  "fix" 
the  object,  though  in  reality  it  is  the  object  which  fixes  the  eye  anc/ 
by  an  involuntary  cerebral  mechanism  imparts  to  it  a  steadiness  fa' 
beyond  any  voluntary  power  to  imitate. 

In  the  dark,  or  with  an  absolutely  homogeneous  field  befon 
them,  the  eyes  are  always  moving.  We  may  infer  this  from  the 
behavior  of  blind  eyes,  and  from  the  fact  that  if  even  one  eye  be 
placed  in  the  dark  (Chapter  XIV)  it  slowly,  though  slightly, 
wanders  from  side  to  side. 

Direct  Vision. — The  value  of  the  steadiness  of  fixation  is  to 
retain  the  image  upon  the  most  highly  differentiated  part  of  the 
retina —  the  ' '  fovea  centralis. ' ' 

Indirect  Vision. — Immediately  outside  the  tiny  area  of  acute 
vision  the  form-sense  (by  which  we  perceive  the  shape  of  objects) 
becomes  reduced  to  one-tenth,  the  reduction  proceeding  still  farther 
with  greater  removal. 

This  "indirect"  vision,  however,  is  as  good,  if  not  better,  than 
' '  direct ' '  vision  for  the  perception  of  light,  so  much  so  that  astrono- 
mers generally  see  a  star  better  by  looking  a  little  to  one  side  of  it. 

Moreover,  in  ' '  indirect ' '  vision  the  eye  is  remarkably  sensitive 
to  the  moving  of  objects,  a  benificent  provision  to  which  we  owe 
many  an  escape  from  danger. 

98 


Fixation,  Projection  and  Binocular  Vision  99 

The  chief  office  of  indirect  vision,  therefore,  is  to  merely  call 
our  attention  to  the  presence  of  objects,  and  especially  moving 
objects,  without  defining  their  shape,  in  order  that  the  eyes  may 
turn  towards  them  and  learn  their  true  nature  by  direct  vision. 

Fixation-Reflex. — Fixation  is  the  result  of  a  visual-reflex  action 
of  extreme  intricacy,  the  center  for  which  lies  probably  in  or  near  the 
corpora  quadri  gemina  ;  we  can  by  an  effort  of  the  will  easily  glance 
from  one  point  to  another,  but  when  we  do  so,  the  new  point 
becomes  that  which  immediately  fixes  the  eye.* 

Persistent  Fixation.— When  the  higher  volitional  centers 
become  weakened  in  any  way,  as,  for  instance,  in  hysteria,  the 
reflex  mechanism  of  fixation  may  even  gain  the  upper  hand  over 
volition,  so  that  it  becomes  difficult  to  transfer  fixation  from  one 
point  to  another.  Dr.  Gowers,  for  instance,  says:  "I  recorded, 
some  years  ago  ("Brain,"  vol.  ii),  a  case  in  which  the  reflex 
fixation  of  the  eyes  was  brought  into  salience  by  disease.  If  the 
patient,  looking  at  one  object,  was  told  to  look  at  another  at  some 
lateral  distance  from  the  first,  his  head  was  instantly  turned  in  the 
direction  of  the  second  object,  but  the  eyes  remained  fixed  on  the 
first  by  a  movement  as  rapid  as  that  of  the  head,  but  in  the 
opposite  direction,  and  then  they  were  slowly  moved  into  the  posi- 
tion corresponding  to  the  second  object.  The  patient  was  in  the 
last  stage  of  progressive  muscular  atrophy,  "f 

I  was  interested  to  note  in  the  record  of  a  case  of  poisoning  by  a 
certain  plant,  the  Cicuta  virosa,  the  following  symptoms:  "She 
stares  with  unaltered  look  at  one  and  the  same  place,  and  cannot  help 
it."  It  may  be,  however,  that  the  symptom  was  not  due  to  any  spe- 
cific action  of  the  plant,  but  to  hysteria  set  in  activity  by  the  poison. 

It  is  not  unlikely  that  one  form  of  "fascination"  depends  on 
this  loss  of  voluntary  control  over  fixation,  though  probably  it  is 
oftener  a  more  purely  psychical  phenomenon.  It  is,  to  use  a  simple 
illustration,  as  though  a  boy  stands  before  a  rock  with  a  limpet  in 
his  hand  :  he  can  choose  any  spot  he  pleases  to  place  the  limpet  on, 
but  now  he  finds  that  to  transfer  it  to  another  spot  is  not  so  easy. 

Lower  Animals. — The  foveal  differentiations  of  human  eyes 
have  to  be  much  more  perfect  than  those  lower  in  the  scale  of 

*Though  in  ordinary  vision  the  "point  of  attention"  (which  engages  the  mind)  becomes 
instantaneously  the  "  point  of  fixation  "  (which  engages  the  eye):  or  rice  rersa,  a  distinction 
must  l>e  observed  between  the  two,  since  we  can,  by  an  effort  of  the  will,  fix  one  point  while 
(lii-eeting  our  attention  to  a  neighboring  one.  We  count,  indeed,  on  the  possession  of  this 
faculty  in  our  patients  when  we  test  their  field  for  colors,  or  their  indirect  vision  by  the 
perimeter. 

t  "  Diseases  of  the  Nervous  System,"  vol.  ii,  p.  195. 


ioo  Tests  and  Studies  of  the  Ocular  Muscles 

creation,  and  it  is  probable  that  in  most  animals  the  whole  retina 
possesses  properties  intermediate  between  the  center  and  periphery 
of  our  own,  so  that  their  sense  of  the  form  of  objects  is  inferior  to 
that  which  direct  vision,  and  superior  to  that  which  indirect  vision 
affords  to  ourselves. 

Central  Fixation. — The  point  of  fixation  is  surrounded  by  an  area 
of  acute  vision  said  to  be  about  three-fourths  inch  in  diameter  at  the 
distance  of  a  foot  (Le  Conte).  For  small  objects,  therefore,  it  may 
suffice  to  fix  one  point,  since  all  the  other  points  will  lie  in  this  area  ; 
but  for  larger  objects  it  is  essential  to  glance  from  point  to  point. 

"The  anatomical  fovea  has  a  breadth  of  0.2  mm.  to  0.4  mm. 
(Henfe),  or,  viewed  from  the  posterior  nodal  point  (which  is 
16  mm.  from  the  retina)  an  angular  breadth  of  45'  to  i°  30'.  On 
looking  at  the  sky  the  fovea  would,  therefore,  cover  a  portion 
having  two  or  three  times  the  diameter  of  the  moon,  which  corres- 
ponds to  half  a  degree.  The  point  of  fixation  has  a  much  smaller 
breadth,  for  we  can  easily  tell  whether  we  are  fixing  the  right-hand 
or  left-hand  margin  of  the  moon.  In  general,  as  soon  as  we  can 
distinguish  that  two  points  are  discrete  we  can  tell  which  we  are 
fixing.  It  wasyatWwho  emphasized  this  fact"  (Tscherning).* 

By  "central  fixation"  we  mean  fixation  exerted  to  bring  the 
images  of  objects  on  the  point  of  acutest  vision,  and  this  is  almost 
the  only  kind  of  fixation  which  exists  in  the  ordinary  use  of  healthy 
eyes  ;  but  since  it  exists  in  the  interests  of  direct  vision,  it  becomes 
lost  as  soon  as  the  power  of  direct  vision  is  destroyed  :  as,  for 
instance,  by  disease  of  the  macula,  or  a  central  scotoma.  The  eye 
then  tends  to  wander,  since  the  central  blind  area  of  the  retina  is 
surrounded  by  a  zone  in  which  no  point  of  acuter  vision  than  the 
rest  exists,  or,  if  it  does  exist,  the  eye  has  yet  to  learn  to  use  it 
exclusively  for  fixation.  If  central  vision  is  impaired  at  birth,  or 
shortly  afterwards,  true  fixation  is  not  often  acquired,  and  nystagmus 
frequently  results.  Ophthalmoscopic  corneal  images,  as  Priestley 
Smith  has  pointed  out,  afford  a  simple  means  of  observing  whether 
an  eye  has  central  fixation  or  not. 

"Fixation-Line."— Having  defined  the  "point  of  fixation  "  as 
that  point  outside  the  eye  which  at  any  moment  engages  the  eye,  it 
is  easy  to  conceive  the  "axis  of  fixation"  or  "fixation-line"  as 
an  imaginary  straight  line  extending  from  this  point  to  the  center 
of  motion  of  the  eyeball. 

*  "  Physiologic  Optics."  p.  36. 


Fixation,  Projection  and  Binocular  Vision 


101 


Field  Of  Fixation. — The  field  of  fixation  is  the  expression  of 
the  mobility  of  the  eye  in  all  directions.  It  is,  for  this  reason, 
sometimes  called  the  ' '  motor  field. ' ' 

If  we  think  of  the  eye  as  placed  in  the  center  of  an  imaginary 
sphere,  the  part  of  the  sphere  which  bounds  the  extreme  sweeps  of 
the  fixation  line  is  the  field  of  fixation. 

Fig-  37  gives  Landolt's  measures  for  the  greatest  possible 
excursions  of  an  eye  in  all  directions,  while  Fig.  38  gives  Schuur- 
man's.  Both  of  these  observers  distinguished  between  the  lateral 
mobility  of  hypermetropic,  myopic  and  emmetropic  eyes.  In  both 


34° 


M.    38 
t.     42° 
H.     38C 


Nose 


Fig  37 

Landolt's  Figures  for  the  Field 
of  Fixation. 


-41°  M. 
-45°  E. 
-47°  H. 


(Nose) 


57° 
Fig.  38 

Schuurman's  Figures  for  the  Field  of 
Fixation  in  Myopia,  Enimetropia 
and  Hypermetropia. 


figures  the  upward  mobility  of  the  eye  is  considerably  less  than  the 
downward.  Stevens  gives  33°  for  the  maximum  elevation  and  50° 
the  maximum  depression  of  normal  eyes.* 

Binocular  Fixation. — It  is  frequently  taken  for  granted  that 
binocular  fixation  and  stereoscopic  vision  are  necessarily  the  same 
thing,  but  the  latter  requires  a  higher  order  of  cerebration  than  the 
former 

On  throwing  the  light  from  the  ophthalmoscope  into  the  eyes 
of  a  patient  after  operating  for  squint,  it  is  not  uncommon  to  find 
that  while  both  eyes  apparently  fix  the  mirror  quite  truly,  yet  the 
subject  of  the  experiment  suppresses  the  image  of  one  eye.  It  is,  of 
course,  very  difficult  to  prove  that  binocular  fixation  under  these 


*To   departures  from   this  proportion  Stevens  has   given  the  names  of  "ancephoria"  or 
"kataphoria,"  according  as  elevation  exceeds  or  falls  short  of  its  proportion  to  depression. 


IO2  Tests  and  Studies  of  the  Ocular  Muscles 

circumstances  is  real  instead  of  apparent,  but  it  is  well  to  bear  in 
mind  the  possibility  of  its  existence. 

Projection. — Objects  whose  pictures  are  formed  upon  the  retina 
are  not  themselves  supposed  to  be  within  the  eyeball,  but  are 
mentally  relegated  to  some  external  position  in  space.  This 
cerebral  process  is  called  "projection."  The  more  perfectly  it  is 
performed  the  more  truly  the  projected  pictures  of  objects  coincide 
with  the  objects  themselves.  Though  projection  is  a  congenital 
faculty,  since  there  never  was  a  time  when  we  imagined  objects  to 
be  located  within  our  eyes,  it  is  perfected  during  the  exercises  of 
childhood  when  the  real  position  of  objects  is  constantly  being 
discovered  by  other  senses. 

Given  the  direction  of  an  object  from  an  eye,  and  its  distance 
in  that  direction,  its  position  is  known.  It  is  convenient,  however, 
to  treat  the  ' '  perception  of  distance "  as  a  separate  study,  and 
treat  projection  as  if  related  only  to  "direction." 

It  is  important  to  recognize  that  projection  is  not  a  faculty  of 
the  retina,  but  is  a  mental  act. 

Field  Of  Projection. — Related  by  its  own  constant  angle  to 
the  "line  of  direction"  in  which  a  picture  focused  on  a  single 
fovea  is  projected,  there  is  a  definite  and  unchangeable  line  of  direc- 
tion belonging  to  every  percipient  element  in  the  retina. 

Though  the  attention  of  the  mind  is  generally  concentrated 
upon  whatever  picture  occupies  for  the  moment  the  fovea,  the 
whole  retina  is  covered  by  a  continuous  sheet  of  pictures  of  other 
objects,  both  near  and  distant,  some  in  and  some  out  of  focus. 
It  is  the  projected  images  of  these  which  constitute  the  field  of 
projection. 

Since  pictures  on  the  retina  are  inverted  and  since  the  direc- 
tion of  projection  coincides  practically  with  the  axes  of  the  incident 
pencils  of  light  which  enter  the  pupil  from  outside  objects  and 
which,  therefore,  cross  each  other  in  the  crystalline  lens,  it  follows 
that  the  field  of  projection  is  re-inverted,  so  that  its  right  half  cor- 
responds to  the  left  half  of  the  retina  and  its  upper  half  to  the 
lower  half  of  the  retina.  For  this  reason  objects,  in  spite  of  their 
retinal  images  being  inverted,  appear  erect  and  as  they  are. 

Malprojection  Of  a  Field. — In  natural  acts  of  vision  the  direction 
of  projection  of  an  image  on  the  fovea  coincides  with  the  visual 
axis  of  the  eye  ;  but  when,  under  some  unusual  or  pathological 
conditions,  the  direction  of  foveal  projection  is  displaced  away  from 


Fixation,  Projection  and  Binocular  Vision  103 

the  visual  axis,  every  part  of  the  field  of  projection  is  equally  dis- 
placed in  the  same  direction,  so  as  faithfully  to  retain  its  relation  to 
the  foveal  projection.  In  other  words,  projection  in  a  field  is 
always  true,*  though  projection  of  •&  field  may  be  "  false." 

Coincidence  of  the  Two  Foveal  Projections. — Pictures  formed 
upon  the  two  foveae  are  projected  under  all  conditions  very  faith- 
fully to  the  same  spot  in  space. 

Thus,  in  recent  paralytic  squint,  two  candles  held  in  line  with 
the  two  visual  axes  invariably  appear  as  one. 

Again,  if  a  piece  of  paper  be  pricked  through  with  a  pin  at 
two  points  separated  by  the  interocular  distance,  and  be  held  up 
close  to  the  eyes  so  that  distant  objects  can  be  seen  through  them, 
the  two  holes  themselves  are  seen  as  one,  in  the  median  line. 

Another  well-known  experiment  is  to  create  a  tiny  foveal  after- 
image by  looking  at  a  bright  point  of  light  with  one  eye  ;  then, 
whatever  object  be  fixed  by  the  other  eye  and  however  much  the 
spectralized  eye  may  be  artificially  displaced  even  by  forceps,  or 
made  to  squint,  the  after-image  still  clings  tenaciously  to  whatever 
point  is  fixed  by  the  other  eye. 

Corresponding  Points. — What  is  true  of  the  fovea  is  also  true, 
in  a  less-pronounced  way,  of  all  other  parts  of  the  retina.  Every 
percipient  element  in  one  retina  has  a  corresponding  element  in  the 
other  (situated  similarly  with  respect  to  its  fovea,  i.  e.,  at  an  equal 
distance  in  the  same  direction  from  it,  so  that  its  projection  in 
space  is  identical). 

Double  Set  of  Corresponding:  Points.— In  very  old-standing 
squint  (strabismus  incongruus)  it  sometimes  happens  that  certain, 
at  least,  of  the  percipient  points  in  one  retina  have  two  correspond- 
ing points  in  the  other,  of  which  one  was  originally  true,  and  is, 
therefore,  again  true  when  the  eyes  are  put  straight  by  operation, 
and  the  other  created  during  the  condition  of  squint. 

Rotation  of  the  Field  of  Projection.— It  has  seemed  to  me 
possible  that  without  any  actual  translation  of  the  field  as  just 
described,  the  corresponding  points  may  become  altered  in  some 
cases  by  rotation  as  a  whole  about  the  point  of  fixation.  In  a  case, 
for  example,  of  complete  traumatic  paralysis  of  the  superior  oblique 
I  found,  years  after,  that  the  field  of  projection  was  still  perfectly 
untorted  on  looking  straight  forward.  Since  the  eye  gave  much 
evidence  otherwise  of  so-called  secondary  contracture  (consecutive 

*This  statement,  ot  course,  supposes  absence  of  anatomical  changes  in  the  lens,  etc. 


IO4  Tests  and  Studies  of  the  Ocular  Muscles 

deviation),  it  can  scarcely  be  conceived  that  in  this  case  there  has 
been  no  actual  paralytic  torsion. 

Physiological  Diplopia. — In  every  ordinary  act  of  vision  a  vast 
number  of  objects  do  not  throw  their  images  on  corresponding 
points  of  the  two  retinae.  For  every  position  of  the  point  of  fixa- 
tion there  is  what  is  called  a  "  horopteric  surface,"  all  objects  in 
which  are  seen  single,  while  all  other  objects  would  be  seen  double 
were  they  closely  analyzed. 

If  the  two  forefingers  be  held  before  the  face  in  the  median 
plane,  one  in  advance  of  the  other,  and  the  farthest  one  be  fixed, 
the  near  one  is  seen  double  ;  and  by  momentarily  closing  the  right 
eye  it  is  easy  to  assure  one's  self  that  the  left  of  the  two  images 
belongs  to  the  right  eye  and  the  right  image  to  the  left  eye.  This 
proximal  diplopia  (as  we  may  call  it)  is,  therefore,  crossed.  (See 
Fig.  40.) 

If,  on  the  other  hand,  the  near  finger  be  fixed,  of  the  two 
images  of  the  distant  one  which  now  appear,  the  right  one  disap- 
pears on  closing  the  right  eye,  and  the  left  on  closing  the  left, 
showing  that  the  distal  diplopia  (as  we  may  call  it)  is  komonymous, 
By  such  experiments,  we  learn  that  all  objects  nearer  to  us  than 
the  point  of  fixation  (and  the  horopteric  surface  connected  with  it) 
have  crossed  images,  while  all  objects  beyond  have  homonymous 
ones.  Our  higher  intellectual  powers  are  insufficient  to  inform  us 
whether  any  double  images  which  we  see  are  crossed  or  homony- 
mous, proximal  or  distal  ;  but  some  inferior  center  seems  to  have 
no  such  difficulty,  for  as  soon  as  an  effort  is  made  to  unite  the 
images,  it  always  commences  in  the  right  direction  without  any 
preliminary  trial  to  discover  whether  it  is  an  effort  of  convergence 
or  one  of  divergence  that  is  called  for. 

Suppression  Of  Images. — In  the  physiological  diplopia  of  people 
who  are  right-handed,  the  image  which  belongs  to  the  right  eye  is 
apt  to  appear  more  substantial-looking  than  the  other  (Tscherning), 
and  this  is  probably  especially  the  case  with  those  who  are  accustomed 
to  frequently  use  the  right  eye  separately,  as,  e.  g.,  in  aiming.  Con- 
sistently with  this  indication,  while  attention  is  diverted  from  the 
diplopia  and  concentrated  upon  the  point  of  fixation,  the  less  sub- 
stantial image  of  objects  out  of  the  horopter  is  in  most  persons  so 
entirely  ignored  by  the  mind  as  to  be  what  is  called  "  suppressed." 
Even  when  it  is  not  so,  the  diplopia  attracts  no  attention,  because 
from  absence  of  critical  analysis  it  is  undistinguished  by  the  mind 


Fixation,  Projection  and  Binocular  Vision  105 

from  that  other  kind  of  indistinctness  which  is  due  to  the  object 
being  out  of  focus. 

Origin  Of  Projection. — In  projecting  the  retinal  field  into  space 
the  mind  must  have  some  "  point  of  origin  "  for  the  radius  vector, 
or  "line  of  direction"  in  which  the  projection  is  made.  Hering 
places  this  origin  midway  between  the  two  eyes,  as  if  they  were 
united  into  one  cyclopic  eye.  His  idea  is  supported  by  the  double 
pin-hole  test  previously  mentioned,  and  is  no  doubt  true  of  those 
whose  eyes  are  of  equal  value  in  binocular  vision.  Some,  however, 
and  possibly  the  majority  even  of  those  who  have  equal  visual 
acuity  in  the  two  eyes,  seem  to  use  one  eye  rather  as  the  aide-de- 
camp of  the  other,  than  as  an  equal  partner  in  projection.  One  is 
then  called  the  "directing  eye"  (Javal),  since  the  origin  of  projec- 
tion appears  to  be  displaced  to  coincide  with  this  eye.  Tscherning 
finds  this  condition  in  his  own  case  and  that  of  several  others,  and 
himself  evidently  judges  of  the  position  of  objects  much  more 
truly  with  his  right  eye  than  with  the  left,  probably  because  it  has 
been  most  often  used  separately. 

Test  for  True  Projection. — The  following  modification  of  a 
very  old  test  by  Hering  enables  the  co-ordination  of  hand  and 
eye  to  be  well  tested  :  Take  a  large  piece  of  cardboard,  marked  in 
the  middle  of  each  surface  with  a  short  vertical  line,  these  lines 
being  exactly  counterposed,  which  can  easily  be  ensured  by 
pricking  the  cardboard  through  at  their  extremities.  Holding 
the  card  vertically  six  inches  before  the  patient's  eyes,  let  him 
endeavor,  by  passing  his  hand  behind  the  cardboard,  to  place  his 
finger  exactly  behind  the  vertical  line  which  he  sees.  He  should 
make  the  attempt  as  carefully  and  judgingly  as  possible,  with  first 
one  eye  shut  and  then  the  other,  using  also  in  each  case  first  the 
right  hand  and  then  the  left.  The  surgeon  standing  behind  the 
cardboard  can  see  perfectly  from  the  line  on  the  back  the  nature  of 
any  failure  in  projection,  while  the  patient  never,  himself  learns  in 
what  direction  his  aim  has  been  missed.  Herein  lies  the  advantage 
of  modification,  for  if  once  the  patient  knows  his  error,  he  makes 
mental  allowance  for  it  in  his  next  attempt. 

Illustrative  Errors  Of  Projection. — In  near  vision,  as  we  may 
see  elsewrhere,  an  excluded  eye  generally  deviates  outwards,  and 
hence  arises  an  error  in  monocular  projection.  Fig.  39  illus- 
trates, as  an  example,  the  case  of  an  aurist  examining  the  drum 
of  an  ear.  The  left  eye,  having  nothing  to  do,  diverges,  and  the 


io6 


Tests  and  Studies  of  the  Ocular  Muscles 


Fig.  39 

Mis-projection  by  an  aurist 
who  closes,  or  does  not 
use,  his  left  eye. 


apparent  position  of  the  drum,  as  represented  in  dotted  outline, 
lies  in  consequence  midway  between  the  visual  lines. 

Another  error  of  projection  may  be  demonstrated  by  quickly 
thrusting  the  finger  at  a  pencil  held  about  a  foot  away  from  the 
eyes  at  the  extreme  lateral  limit  of  the  motor 
field  while  the  eyes  are  strongly  turned 
toward  it.  The  finger  will  generally  miss 
its  mark  to  the  outer  side  of  the  pencil.  The 
reason  of  this  error  is  that  the  ordinary  cal- 
culations of  the  mind  are  formed  from  the 
more  habitual  smaller  obliques  of  vision,  and 
the  excessive  effort  required  to  produce 
unusual  obliquity  of  the  visual  axes  creates 
the  impression  of  a  proportionately  displaced 
object.  At  the  limits  of  the  motor  field, 
strong  increments  of  effort  produce  smaller 
increments  of  result,  owning  to  mechanical 
difficulties  in  the  motions  of  the  eyes. 

Malprojection,   kindred  to    the   last,    is 
seen  to  a  more  marked  extent  when  a  muscle 

is  paralyzed.  Since  the  mind  is  counting  on  every  muscle  to  do  its 
duty,  the  least  failure  in  contractile  response  to  stimulus  results  in 
malprojection  proportionate  to  the  failure,  and  in  the  direction 
which  is  suggested  by  the  greater  effort  put  forth. 

Fusion. — Since  corresponding  points  have  their  pictures  pro- 
jected to  the  same  point  in  space,  the  mind  cannot  but  regard  them 
as  one,  since  it  cannot  conceive  two  objects  occupying  the  same 
place  at  the  same  time. 

But  there  is  to  be  considered  more  than  the  mere  existence  of 
single  vision  :  there  is  a  natural  love  of  single  vision,  expressed  by 
a  strong  sub-conscious  desire  to  bring  together  and  thus  fuse  any 
double  images  of  the  same  object  while  even  one  of  them  engages 
the  attention  of  the  mind.  Withdrawal  of  attention  to  another 
object  almost,  if  not  quite,  abolishes  this  desire  ;  also  anything 
which  makes  one  image  differ  from  another,  either  in  color,  size 
or  shape. 

It  is  the  absence  of  this  "abhorrence  of  double  images"  or 
"love  of  single  vision,"  as  it  has  been  called,  in  very  long-stand- 
ing cases  of  strabismus  which  is  the  chief  difficulty  encountered  in 
training  the  eyes  to  work  again  together.  When  entirely  absent, 


Fixation,  Projection  and  Binocular  Vision  107 

it  is  said  that  there  may  be  even  a  desire  to  separate  the  images 
in  order  to  see  one  of  them  more  clearly,  a  condition  described 
by  Graefe  as  antipathy  to  single  vision.  But  this  antipathy  is, 
I  fancy,  merely  a  mental  choice,  not  a  sub-conscious  contrast 
to  the  "love."  It  is  quite  reasonable  to  expect  it,  since  one 
field  embarrasses  the  other  less  in  proportion  to  its  displacement 
therefrom. 

The  Power  Of  Overcoming:  Prisms.— If  a  prism  be  held  before 
one  eye,  its  effect  will  be  to  displace  the  image  belonging  to  that 
eye  to  another  part  of  the  retina,  so  that,  for  a  moment,  vision  will 
be  double.  The  image  seen  by  the  naked  eye  is  the  "true" 
image,  since  it  is  mentally  referred  to  its  true  position  in  space  : 
the  image  seen  through  the  prism  appears  displaced  from  the  true 
position  of  the  object  in  the  direction  of  the  apex  of  the  prism  by 
an  angular  departure  equal  to  the  angle  by  which  the  prism  deviates 
light,  and  which  for  brevity  we  call  the  "deviating  angle  of  the 
prism."  If  the  prism  be  weak  enough,  the  co-ordinating  centers 
endeavor  to  overcome  the  diplopia  by  directing  the  embarrassed  eye 
towards  the  apex  of  the  prism,  so  as  to  again  receive  its  image  on 
the  fovea.  This  is  done  by  the  conspiracy  of  at  least  two  conjugate 
innervations,  and  when  it  is  effected,  vision  is  again  single.* 

Apparent  Prismatic  Displacement. — Now,  however,  the  object 
does  not  appear  to  occupy  the  position  either  of  the  former  true 
one  or  of  the  former  false  one,  but  lies  exactly  midway  between  the 
two.  A  person  with  both  eyes  open  and  a  prism  before  one  eye, 
will,  as  I  have  shown  elsewhere,  misjudge  the  position  of  objects, 
even  though  he  see  them  single  ;  but  his  malprojection  will  only 
equal  half  the  deviating  angle  of  the  prism.  If  he  cover  the  naked 
eye  with  his  hand,  under  these  circumstances,  the  image  may 
appear  to  move  slowly  till  the  malprojection  is  doubled.  All  this 
proves  that  the  innervations  at  play  are  conjugate,  according  to  the 
principles  already  mentioned. 

\Yhen  distant  objects  are  viewed,  a  prism  higher  than  No.  8 
(with  4°  deviation  therefore  ;  for  the  deviation  of  light  by  a 
prism  is  by  an  angle  about  half  its  apical  angle),  with  apex  out, 
cannot  generally  be  overcome. 

Since  the  prism  is  held  before  one  eye,  we  have  to  mentally 
divide  its  effect  between  the  two  eyes.  It  follows,  therefore,  that  a 

*For  diagrams  to  illustrate  this,  vide  "  Ophthalmological    Prisms"    (J.  Wright  &  Co.), 
pp.  62,  63. 


io8  Tests  and  Studies  of  the  Ocular  Muscles 

divergence  of  2°  from  parallelism,  of  each  eye,  is  the  greatest  diver- 
gence which  even  the  love  of  single  vision  can  usually  induce  the 
co-ordinating  centers  to  effect.  It  is  far  otherwise,  however,  with 
prisms  whose  apices  are  placed  inwards,  and  which  can  be  increased 
to  much  greater  strengths  without  inducing  diplopia.  Since,  in  the 
vertical  motions  of  the  eyes,  there  is  nothing  known  correspond- 
ing to  the  converging  innervation,  it  is  remarkable  that  prisms 
with  their  apices  upwards  or  downwards  can  be  overcome  at  all. 
As  a  matter  of  fact,  to  be  overcome,  they  must  be  very  weak. 
A  prism  of  from  2°  to  4°  (i°  d.  to  2°  d.)  before  one  eye,  is  the 
strongest  vertical  prism  that  can  generally  be  overcome,  without 
practice. 

Breadth  Of  Fusion  Power. — When  diplopia  is  created  artificially, 
as  by  prisms,  the  smaller  it  is,  i,  e.,  the  less  the  double  images 
are  separated,  the  greater  is  the  desire,  and  the  easier  is  the 
task,  to  effect  fusion.  There  are  limits  to  the  separation  of 
the  images,  beyond  which  the  diplopia  becomes  insuperable. 
These  limits  define  the  ' '  breadth  of  fusion ' '  (as  it  is  generally 
called  for  brevity). 

Three  Conditions. — When  the  breadth  of  diplopia  is  ( i )  greater 
than  the  breadth  of  fusion  power,  no  effort  can  unite  the  images. 
When  they  are  (2)  almost  equal,  the  images  may  be  united  by  a 
great  effort  for  a  short  time.  When  the  breadth  of  diplopia  is  con- 
siderably (3)  less  than  the  breadth  of  fusion  power,  the  images  are 
easily  united.  These  three  conditions  are  found  respectively  in 
"permanent  squint, "  in  "periodic  squint"  and  in  "  latent  squint" 
(heterophoria).  The  difference  between  them  is  merely  a  question 
of  degree. 

There  is  a  great  difference  between  the  breadth  of  fusion 
power  in  different  individuals,  and  it  varies  also  for  different  dis- 
tances of  the  object-point,  and  according  as  the  diplopia  is  homony- 
mous  or  crossed  ;  according,  too,  as  the  health  and  the  will- 
power vary. 

The  power  to  fuse  horizontally  separate  images  is  much  greater 
than  the  power  to  fuse  those  which  are  separated  vertically  ;  and 
"crossed"  diplopia  is  more  easily  overcome  than  "homonymous," 
since  converging  effort  is  easy  and  diverging  effort  difficult. 

The  best  way  to  estimate  the  breadth  of  fusion  is  to  find  the 
strongest  prisms  which  the  eyes  can  overcome  :  the  strongest  prism 
base  in  added  to  the  strongest  prism  base  out,  gives  the  horizontal 


Fixation,  Projection  and  Binocular  Vision  109 

breadth,  while  the  strongest  prism  base  down  added  to  the  strongest 
prism  base  up,  gives  the  vertical  breadth. 

A  convenient  convention  to  adopt  is  that  prisms  base  in  measure  the 
negative  breadth  of  fusion,  and  that  prisms  base  out  measure  the  positive 
breadth  :  the  two  together,  of  course,  constitute  the  total  amplitude. 

In  making  tests  of  this  kind  with  prisms  it  is  necessary  to 
remember  that  anything  which  makes  one  image  differ  from  the 
other  lessens  the  desire  to  unite  them  ;  hence,  in  using  strong  prisms 
which  alter  the  image  by  chromatic  and  prismatic  aberration,  it  is 
best  to  divide  them  equally  between  the  two  eyes,  so  that  the  two 
images  shall  be  equally  perturbed. 

Caution. — In  cases  of  defective  converging  power  at  reading 
distance  some  have  tenotomized  the  external  rectus  with  the  result 
of  producing  homonymous  diplopia  in  distant  vision.  Such  mistakes 
would  have  been  avoided  by  taking  the  trouble  to  test  how  much 
negative  breadth  of  fusion  the  patients  possessed  (or,  in  other 
words,  how  strong  a  prism,  base  in,  they  could  overcome)  in  dis- 
tant vision.  A  defective  negative  breadth  is  an  evident  contra- 
indication against  division  of  the  external  rectus  tendon,  for 
homonymous  diplopia  in  distant  vision  is  the  most  difficult  of  all 
the  horizontal  forms  of  diplopia  to  overcome. 

Binocular  Fixation.— We  should,  perhaps,  draw  a  distinction 
between  (a)  Binocular  fixation,  (^)  Binocular  vision  and  (c)  Stereo- 
scopic vision  or  perception  of  relief.  The  first  of  these  is  beneath 
the  region  of  consciousness,  the  two  eyes  jointly  fixing  the  same 
object  from  habit,  even  when  the  mind  suppresses  or  at  least  pays 
no  regard  to  the  vision  of  one  eye.  I  have  seen  cases  in  which  it 
seemed  that  binocular  fixation  was  preserved,  both  visual  axes 
being  directed  correctly,  so  far  as  objective  tests  could  discover, 
even  though  diplopia  could  not  be  elicited  by  prisms.  However 
that  may  be  (for  it  is  confessedly  difficult  to  understand),  there  is 
no  doubt  that  the  second  should  be  distinguished  from  the  third, 
for  binocular  vision,  in  which  certain  objects  seen  by  one  eye  are 
mentally  recognized  simultaneously  with  vision  by  the  other  eye,  is 
inferior  to  the  powrer  of  erecting  bodies  into  ' '  relief ' '  with  such  an 
instrument  as  the  stereoscope.  Some  have  this  last  power  much 
more  intensely  than  others. 

Monocular  Perception  of  Distance.— It  is  well  to  know  in  how 
many  ways  a  single  eye  can  gain  an  idea  of  the  third  dimension,  so  as 
not  to  be  deceived  when  testing  for  true  binocular  vision.  They  are  : 


no  j  esis  and  Studies  of  the  Ocular  Muscles 

(*z)  Aerial  perspective. — More  distant  objects  are  veiled  by  a 
greater  depth  of  atmosphere,  and  the  greater  the  depth  of  atmos- 
phere the  bluer  also  this  veil  is.  In  mountainous  districts,  when 
the  atmosphere  is  unusually  clear,  distances  are  judged  to  be  less 
than  they  really  are  ;  the  reverse  being  the  case  in  a  fog. 

(6)  Shadows  and  overlappings. — With  the  source  of  light 
behind  us,  an  object  which  throws  its  shadow  on  another  object  is,  of 
course,  nearer.  So  also  is  an  object  which  hides  part  of  another 
object. 

(r)  Visual  angle  of  known  objects. — The  size  of  many  objects 
is  so  well  known,  that  their  distance  can  be  estimated  by  their 
apparent  magnitude,  as,  for  instance,  in  the  case  of  men,  horses,  etc. 

(*/)  Mathematical  perspective. — The  gradual  decrease  in  the 
size  of  similar  objects  and  the  gradual  approximation  of  parallel 
lines,  is  too  well  known  to  need  further  description  here.  The 
number  of  intervening  objects  also  influences  our  judgment  ;  hence, 
distances  at  sea  appear  less  than  on  land,  it  is  stated. 

(<?)  focal  indistinctness. — The  farther  objects  lie  from  the  point 
of  distinct  vision,  on  the  far  or  near  side,  the  more  hazy  they  are. 

(/")  Accommodation.  —  It  is  only  in  judging  of  comparatively 
near  objects  that  any  assistance  is  derived  from  the  conscious  effort 
of  accommodation.  As  a  rule,  a  greater  effort  of  accommodation 
makes  us  think  objects  to  be  smaller. 

(  £•)  Parallax. — For  objects  which  are  not  too  distant,  this  is 
by  far  the  most  important  and  valuable  indication  to  a  single-eyed 
person.  Though  he  cannot  see  the  object,  as  others  can,  from  two 
points  of  view  simultaneously,  he  can  do  so  consecutively,  by 
moving  his  head  from  one  position  into  another.  He  sees, 
therefore,  as  we  should,  if  first  one  eye  were  active  and  then 
the  other.  Such  movements  of  the  head  have  to  be  guarded 
against  in  some  of  the  clinical  tests  for  binocular  vision. 

Binocular  Perception  of  Distance.— Here,  in  addition  to  the 
criteria  just  enumerated,  convergence  comes  into  play,  as  well  as 
the  fact  that  the  nearer  an  object  is,  the  more  dissimilar  are  its  two 
pictures  upon  the  two  retinae,  thus  altering  the  character  of  the 
physiological  diplopia.  The  absolute  amount  of  convergence 
exerted  is  not  a  great  help,  but  to  any  volitional  increase  or 
decrease  of  it  the  mind  is  very  sensitive.  The  nearer  an  object, 
the  greater,  of  course,  the  convergence  it  requires,  and  by  con- 
vergingy>w;z  one  object  to  another  we  learn  their  relative  distances. 


Fixation,  Projection  and  Binocular  Vision 


in 


Fig.  40 

To   show   that    Proximal    Diplopi 
crossed,  and  Distal  Diplopia 
homonvmous. 


a    is 


Stereoscopic  Vision  or  Perception  of  Relief.— It  has  been  said 
by  Dove  and  others  that  objects   appear  solid   when  seen   by  so 
instantaneous  an  illumination  as  that  of  an  electric  spark.      If  this 
be   so,   the  appearance    of   solidity 
must  be  due    to    the    physiological 
diplopia    of   those    parts  which  are 
not  seen  single.     For  the  analysis, 
however,  of  "relief"  and  the  quan- 
titative   perception    of  depth,    it   is 
necessary  that  the  eyes  should  unite 
in  succession  different  parts  of  the 
object  by  consecutive  increase  and 
decrease  of  convergence  (Briicke). 
Hence,  many  find  that  with  a  stereo- 
scope the   appearance   of  relief  does  not  appear    until  after   a  few 
such  motions  have  been  made.      Fig.  40  shows  how  this  principle 
works  in  the  case  of  a  lead  pencil,   held  pointing  forwards  in  the 

median  plane  a  little  lower  than 
the  eyes.  When  the  far  end  of 
the  pencil  is  fixed,  the  near  end  is 
seen  double.  By  converging  a 
little  more  so  as  to  fix  the  middle 
of  the  pencil,  both  ends  exhibit 
diplopia  of  half  the  magnitude 
which  the  near  end  at  first  ex- 
hibited, and  by  converging  still 
more  to  look  at  the  near  end  the 
far  end  exhibits  wide  diplopia. 

Stereoscope. — Fig.   41  gives 

the  plan  of  a  Brewster's  stereo- 
scope, A  and  B  being  the  pic- 
tures. These  are  taken  from 
slightly  different  points  of  view 
with  a  photographic  camera,  so 
that  the  distance  between  identical 
objects  in  the  foreground  of  the 

two  pictures  is  less  than  between  identical  objects  in  the  back- 
ground. To  fuse  the  former,  therefore,  more  convergence  is  called 
for  than  to  fuse  the  latter.  Foreground  objects  and  background 
objects  cannot  both  be  fused  simultaneously  :  if  they  could  the 


Fig.  41 

Plan  of  an  ordinary  Stereoscope  (the  pic- 
tures, howpTer,  being  separated  mote 
than  usual  for  diagrammatic  purposes). 


112 


Tests  and  Studies  of  the  Ocular  Muscles 


sensation  of  relief  would  disappear.  While  looking  at  the  fore- 
ground of  the  scene  depicted,  there  is  physiological  diplopia  of 
the  background,  and  vice  versa,  just  as  with  the  lead  pencil  of 
Fig.  40.  The  decentering  outwards  of  the  lenses  enables  the  eyes 
to  converge  somewhat,  as,  for  instance,  to  C ;  but  the  united 
picture  is  generally  only  projected  to  D,  since  the  knowledge 
of  the  convergence,  which  acting  alone  would  project  it  to  C, 
is  in  part  overborne  by  the  conscious  knowledge  of  the 
size  of  the  stereoscope  which  tends  to  bring  the  projection 
towards  the  plane  of  A  B,  unless  the  picture  itself  is  one  of 
a  scene  we  are  accustomed  to  think  of  as  distant.  In  that  case 
the  projection  depends  a  good  deal  on  the  powers  of  imagination 
which  make  the  stereoscope  forgotten.  The  pictures  (A,  B) 
generally  lie  slightly  within  the  focal  length  of  the  lenses,  so  that 
accommodation  is  not  wholly  relaxed,  and  there  is  nearly  always 
a  certain  amount  of  associated  convergence.  The  distance  between 

the  pictures  is  generally 
made  greater  than  the 
inter-ocular  distance,  to 
allow  room  for  larger  pic- 
tures to  be  used.  A  handy 
form  of  cheap  stereoscope 
is  shown  in  Fig.  42.  Of 
the  expensive  ones,  prob- 
ably the  best  for  clinical 
purposes  is  Javal's  "  St6- 
r6oscope  a  cinq  mouve- 
ments. " 

So    various   are  the 

experiments  which  can  be  made  with  a  stereoscope  that  the 
interested  reader  is  referred  to  some  book  which,  like  Javal's, 
is  wholly  devoted  to  the  subject.  One  of  the  best  devices  is 
that  by  Green,  in  which  the  letter  L  is  placed  before  one  eye 
and  a  letter  F  before  the  other.  The  patient  who  uses  both  eyes 
simultaneously  sees  them  combined  into  an  E.  This  is  a  test 
for  binocular  vision,  but  not  for  stereoscopic  vision.  No  stereo- 
scopic test  for  the  notion  of  relief  is  quite  so  clinically  satisfactory 
as  Hering's  drop  test,  for  in  most  others  we  have  to  rely  upon 
the  patient's  statements,  without  being  able  to  verify  them  in  the 
same  unmistakable  wav. 


Fig.  43 


Fixation,  Projection  and  Binocular  Vision  113 

Mr.  Berry's  Sterescope. — This  is  a  very  ingenious  and  satisfac- 
tory arrangement.  Before  each  eye,  in  a  stereoscope  is  placed  a  fixed 
circle,  with  a  small  movable  circle  within  it,  as  shown  in  Fig.  43. 
By  a  simple  mechanism  the  two  small  circles  can  be  made  to  mutually 
approach  one  another,  as  shown  by  the 
small  continuous  circles,  or  mutually 
recede  from  one  another  to  occupy 
the  position  shown  by  the  small  dotted 
circles.  When  their  separation  from 
each  other  is  at  its  least,  they  resemble 
the  images  in  the  foreground  of  a 
landscape,  so  that  the  device  is  seen  Mr  Bcrry8;  sterescope. 

in  relief,   like  a  truncated  cone   or  a 

bucket  upside  down.  But  when  their  separation  increases,  so  as  to 
be  greater  than  the  separation  of  the  large  circles,  they  resemble 
images  in  the  background  of  a  landscape  and  produce  the 
appearance  of  a  hollow  cone  or  empty  bucket.  During  the  motion 
from  one  position  to  the  other  the  stereoscopic  effect  is  one  of  move- 
ment in  the  third  dimension,  the  small  circle  appearing  to  sink  from  a 
plane  above  the  great  one  to  one  which  lies  beneath  it.  This  appa- 
rent movement,  says  Mr.  Berry,  is  so  evident,  especially  if  the  experi- 
ment be  made  in  semi-darkness,  that  young  children  can  at  once  say 
whether  they  see  it  or  not,  and  seeing  it,  of  course,  implies  the  exercise 
of  stereoscopic  vision. 

Lecture  Controlled. — Javal's  long-known  plan  of  holding  a 
pencil  vertically  midway  between  the  patient's  eyes  and  a  page  of 
print  to  see  whether  he  can  read  continuously  without  suddenly 
bobbing  his  head  to  avoid  the  pencil,  is  a  test  not  of  stereoscopic 
vision  nor  even  exactly  of  single  binocular  vision,  but  of  the  power 
of  rapid  alternate  binocular  vision. 

(#)  If  one  eye  be  amblyopic  he  cannot,  of  course,  read  that 
part  of  the  print  which  lies  behind  the  pencil,  as  viewed  from  the 
good  eye,  without  bobbing  his  head. 

(b}  If  both  eyes  have  sufficient  visual  acuity  and  yet  are  not 
working  together,  there  must  either  be  a  head-bobbing  or  else  a 
pause  from  disconcertment  when  the  deviated  eye  has  to  suddenly 
take  up  fixation,  followed  immediately  by  a  second  pause  before 
the  sound  eye  can  resume  it.  An  excellent  arrangement  by 
George  Bull  enables  the  patient  to  place  against  his  forehead  a 
light  framework  which  supports  both  the  print  and  a  vertical  rod  in 


Tests  and  Studies  of  the  Ocular  Jlfuscles 


front  of  it.  Previous  to  this  he  employed  a  bent  strip  of  brass  to  be 
held  against  the  book  by  a  wooden  spring-forceps.  A  somewhat 
similar  arrangement,  but  to  be  held  by  the  thumb  only,  has  been 
used  by  Priestley  Smith.  What  I  use  myself  is  a  Holmes  stereo- 
scope with  two  crosspieces,  one  for  holding  the  print,  the  other  for 
holding  a  series  of  upright  strips  of  metal  or  whalebone,  the  strips 
being  made  of  a  dull  black.  Javal,  too,  has  constructed  a  ' '  multiple 
controller"  consisting  of  five  bars  side  by  side. 

Bering's  Drop  Test. — In  this  test  the  patient  sees  an  object  for 
so  brief  an  interval  that  there  is  scarcely  time  for  a  full  movement 
of  convergence  to  occur.  It  tests,  therefore,  rather  what  has  been 
called  the  "notion"  of  relief,  than  the  "measurement"  of  it.  It 
requires  a  flattened  cylinder  or  shallow  rectan- 
gular wooden  box  about  ten  inches  long  by 
three  or  four  broad,  and  open  at  both  ends. 
From  the  farther  end  two  wires  project  for- 
wards and  outwards,  connected  at  their  ex- 
tremities by  a  horizontal  thread  which  is  pro- 
vided with  a  small  bead  at  its  mid-point  for 
the  patient  to  look  at  through  the  cylinder. 
Fig.  44  shows  a  very  satisfactory  home-made 
arrangement  consisting  of  two  cylinders  of 
cardboard  fixed  together,  the  only  disadvan- 
tage of  which  is  that  the  two  circular  extre- 
mities are  apt  to  solicit  their  own  fusion  and 
thus  interfere  with  the  free  movements  of  con- 
vergence. Whatever  form  is  used,  it  is  impor- 
tant to  exclude  all  vision  of  the  operator's  hands.  Small  objects, 
such  as  beans  or  marbles,  of  different  sizes,  are  dropped  from 
one  hand  into  the  other,  some  beyond  the  thread  and  others  within 
it,  taking  care  that  on  the  whole  those  which  fall  beyond  the  thread 
are  a  little  larger  than  those  which  fall  within  it.  If  stereoscopic 
vision  exist,  he  will  almost  always  give  a  correct  answer  to  the 
question  on  which  side  of  the  string  the  ball  falls  ;  but  if  not,  nearly 
half  the  answers  will  be  wrong. 


Fig.  44 

Home-made  form  of  Ber- 
ing's Drop  Test. 


CHAPTER   VII 


Strabismus 

Definition. — Strabismus  may  be  briefly  defined*  as  "  inconcert 
of  the  fixation  lines,"  or  as  "a  defection  of  one  fixation  line  from 
the  other.  It  exists  whenever  the  two  visual  axes  are  not  directed 
simultaneously  to  the  point  of  fixation.  Only  one  fixation  line 
deviates  as  a  rule,  and  the  angle  of  its  defection  measures  the 
squint. 

Chief  Division. — The  chief  division  of  true  squints  is  into 
paralytic  and  non-paralytic.  This  division  is  almost  identical  with 
that  into  incomitant  and  comitant  squints,  since  in  nearly  all  paralytic 
squints  the  conjugate  movements  of  the  eyes  are  incomitant,  z.  e. , 
are  unequal  in  certain  directions  of  vision,  as  evidenced  by  increas- 
ing separation  of  the  double  images  ;  while,  on  the  other  hand,  in 
nearly  all  non-paralytic  squints  their  equality  is  so  preserved  that 
the  squint  remains  of  the  same  magnitude  in  whatever  direction 
the  eyes  look,  provided  accommodation  remains  unchanged.  We 
shall  see,  too,  further  on,  that  in  paralytic  squints  the  "secondary  ' 
deviation,  i.  e. ,  that  of  the  better  eye  when  it  is  placed  behind  a 
screen  so  as  to  oblige  the  squinting  eye  to  take  up  fixation,  is 
greater  than  the  primary,  while  in  non-paralytic  squints  they  are 
equal.  (Paralytic  squints  are  treated  in  the.  next  chapter.) 

Horizontal  or  Vertical. — When  an  eye  squints  in  or  out,  the 
squint  is  horizontal  and  is  called  "strabismus  convergens,"  or 
"divergens,"  as  the  case  may  be.  When  an  eye  squints  up  or 
down,  the  case  is  one  of  vertical  squint  and  may  be  "  s.  sursumver- 
gens  "  or  "  s.  deorsumvergens,"f  according  as  the  squinting  eye 
is  higher  or  lower  than  its  fellow.  Horizontal  and  vertical  elements 
very  frequently  co-exist,  and  it  is  rare  to  find  a  pronounced  old 
convergent  squint  that  has  not  a  slight  vertical  element  as  well. 

Alternating:  or  Unilateral. — In  the  first,  alternating  squint,  the 
patient  fixes  with  either  eye  at  pleasure,  the  other  squinting  while 


*lt  will  be  seen  that  I  have  not  felt  able  to  adopt  one  author's  suggestion  to  make  defect  ol 
the  fusion  faculty  a  necessary  part  of  the  definition  of  squint.  To  do  so  would  make  the 
definition  far  too  narrow  and  leave  unprovided  for  several  varieties  of  squint  due  to  quite 
other  causes. 

tl  prefer  the  more  manageable  terms,  "s.  ascendens"  and  "s.  descendens,"  but  have 
retained  those  in  the  text  in  deference  to  usage. 

115 


Ii6  Tests  and  Studies  of  the  Ocular  Muscles 

he  does  so,  for  the  reason  that  the  two  eyes  are  of  such  equal 
value  that  he  has  no  preference. 

Worth  finds  that  fifteen  per  cent,  of  constant  squints  belong 
to  the  alternating  variety  and  divides  them  into  "accidentally 
alternating  squints"  and  "essentially  alternating  squints."  The 
first  class  only  differs  from  monolateral  squints  in  the  accident  of 
the  eyes  being  of  equal  refraction.  The  second  class  has  a 
congenital  total  inability  to  acquire  fusion.  Since  there  is  no 
' '  anopsia ' '  in  alternating  squints,  there  is,  of  course,  no 
' '  amblyopia  ex  anopsia. ' '  Alternating  squints  of  the  '  'essential ' ' 
class  are,  of  course,  only  capable  of  cosmetic  correction. 

A  large  number  of  squints  are  transitions  between  the  com- 
pletely alternating  and  the  completely  monolateral  varieties,  one  eye 
squinting  very  much  more  than  the  other,  but  not  exclusively. 
Needless  to  say,  even  the  occasional  use  of  the  generally  squinting 
eye  greatly  retards  the  development  of  its  amblyopia,  though  there 
is  little  doubt  that  the  longer  such  a  squint  is  neglected  the  more  it 
tends  to  become  completely  monolateral. 

In  contrast  to  squints  of  this  kind,  in  which  either  eye  takes 
up  fixation  indifferently,  most  squints  are  "unilateral,"  the  patient 
having  a  distinct  preference  for  one  as  the  "working"  eye.  The 
way  to  distinguish  to  which  of  these  classes  a  squint  belongs,  is  to 
screen  the  working  eye  ;  this  makes  the  other  take  up  fixation. 
If,  on  unscreening,  the  transference  continues  unchanged,  the  squint 
is  alternating  ;  if,  however,  the  squint  reverts  to  its  original  eye,  it  is 
unilateral.  In  unilateral  squints  the  squinting  eye  is  nearly  always 
determined  by  some  diminution  of  visual  acuity,  either  retinal  or 
from  higher  ametropia,  astigmatism  or  corneal  nebulae,  conditions 
which  always  predispose  to  the  development  of  squint.  Traumatic 
cataract  and  macular  hemorrhage  are  mentioned  by  Percival. 

Strabismus  Convergens  Concomitans. — The  great  majority  of 
convergent  squints  are  of  this  kind,  being  purely  due  to  excessive 
activity  of  the  converging  innervation. 

Nearly  all  cases  of  concomitant  convergent  squint  disappear 
under  chloroform,  showing  that  the  internal  recti  are  not  contrac- 
tured  or  structurally  altered,  but  only  unduly  innervated.*  In  most 
cases  this  activity  was  at  first  occasioned  simply  by  association  with 
excessive  accommodative  effort  called  forth  either  by  hypermetropia 


*  I  have  seen  one  case,  but  only  one,  in  which  the  eyes  (previously  divergent)  converged 
under  chloroform. 


Strabismus  117 

or  possibly,  in  a  few  cases,  by  paresis  of  the  ciliary  muscle,  as  sug- 
gested by  Javal.  Convergent  concomitant  squint  is  sometimes  con- 
genital, but  far  more  frequently  commences  about  the  age  of  three 
years,  when  children  first  begin  to  regard  small  objects  attentively. 
Possibly  at  this  age  accommodation  begins  to  require  a  greater 
effort  than  before,  from  changes  in  the  consistency  of  the  lenses  or 
a  diminution  of  its  early  rotundity.  Or,  it  may  be,  that  sometimes 
at  the  age  when  a  squint  begins,  the  insulation  between  accommo- 
dation and  convergence  is  still  more  incomplete  than  usual,  so 
that  strong  accommodation  is  impossible  without  equally  strong 
associated  convergence  from  overflow  of  nervous  force.  The  fre- 
quent association  of  squint  with  some  other  defects  of  the  nervous 
system  has  been  pointed  out  in  France. 

It  has  always  been  a  difficult  question  why  some  hypermetropes 
squint  and  many  others  of  similar  refraction  do  not.  Later  develop- 
ment than  usual  of  the  insulation  just  spoken  of  is  an  extremely 
probable  cause,  while  congenital  deficiency  or  late  development  of 
the  love  of  single  vision  or  feeble  intensity  of  the  fusion  faculty  is 
another  possible  cause,  of  which  we  must  take  an  equal  account. 
The  success  obtained  by  Worth  in  training  the  fusion  faculty  in 
very  young  squinters  shows  how  rarely  the  faculty  is  completely 
absent.  It  proves,  therefore,  that  other  causes  must  co-exist  in  the 
great  majority  of  cases,  since  it  would  be  extremely  unlikely  that 
after  using  the  fusion  faculty  for  three  years  it  would  be  surrendered 
at  the  usual  age  when  squint  comes  on,  unless  its  surrender  were 
compensated  for  by  some  other  gain.  It  is  evident  that  any  squint 
due  solely  to  defect  of  the  fusion  faculty  would  date  from  infancy. 
When  once  formed,  a  squint  persists  from  innervational  habit. 

The  influence  of  habit  is  seen  in  the  fact  that  accommodative 
squints  are  generally  not  lessened  by  as  many  meter  angles  as  there 
are  diopters  of  refracting  power  in  the  correcting  lenses  (Berry). 
This  is  proved  by  measuring  the  squint  first  with,  and  then  without, 
correction. 

Since  the  innervation  is  common  to  the  two  eyes,  it  affects 
them  both  equally,  and  only  the  desire  for  fixation  keeps  them  both 
from  squinting.  When  one  eye  looks  straight  forwards,  in  order 
to  fix  an  object,  doing  so  doubles  the  squint  in  the  other  eye,  so 
that  one  eye  bears  the  blame  for  the  squint  in  both. 

A  squint  is  often  increased  temporarily  by  nervous  excitement — 
a  fact  which  also  points  to  its  innervational  character.  This 


1 1 8  Tests  and  Studies  of  the  Ocular  Jlfuscles 

nervous  element  must  be  distinguished  from  the  accommodative 
element.  In  some  cases  emotion  seems  to  excite  certain  oculo- 
motor centers  more  than  others,,  so  that  a  squint  is  temporarily 
increased  under  its  influence.  This  increase  is  not  necessarily  an 
increase  of  convergence  only,  but  if  there  be  already  a  vettical 
element  in  the  squint,  that  too  may  increase  under  the  influence  of 
emotion.  The  surgeon's  measurements,  therefore,  may  lead  him  to 
form  an  exaggerated  opinion  of  the  squint,  since  it  becomes  greater 
during  consultation.  Happily  these  cases,  in  their  marked  forms, 
are  rather  on  the  rare  side.  They  should  be  approached  with  great 
caution,  if  the  question  of  operation  has  to  be  considered.  It  is  well 
known  too  how  frequently  such  reflex  irritation  as  helminthiasis 
accounts  entirely  for  a  temporary  squint,  and  I  have  proved  that 
even  slight  irritation  of  the  primes  vite  from  indigestion  may  stimu- 
late the  converging  center  enough  to  cause  a  temporary  latent 
squint.  Another  cause  of  temporary  squint  is  hysteria  ;  cases  of 
this  kind  are,  however,  more  frequently  classified  under  the  name 
"spasm  of  convergence." 

Accommodative  Squint. — This  name  is  given  to  a  squint  which 
disappears  during  a  vacant  stare,  appears  when  attention  is  fixed 
and  increases  markedly  as  an  object  of  fixation  is  made  to  approach 
the  eye.  In  its  incipiency  every  accommodative  squint  was  at  first 
only  occasional,  occurring  during  close  vision  of  near  objects,  and 
therefore  (Javal)  likely  to  be  unnoticed,  owing  to  the  inclined  posi- 
tion of  the  head.  The  child,  finding  that  by  allowing  the  squint 
to  occur,  he  can  see  distinctly  with  less  effort,  forms  the  habit  of 
squinting  more  and  more.  Accommodation  is  effected  more  easily 
when  supported  by  a  full,  or  more  than  full,  share  of  associated 
convergence.  Once  formed,  the  habit  of  thus  assisting  the  accom- 
modation cannot  be  broken,  a  new  relation  is  formed  between  the 
two  efforts,  and  the  squint  becomes  less  and  less  confined  to  near 
vision.  At  this  stage,  even  though  in  distant  vision  squint  should 
never  actually  occur,  the  tendency  to  it  is  evidenced  by  the  way 
in  which  an  eye  deviates  inwards  as  soon  as  it  is  screened  by 
Javal's  disk  of  ground  glass,  or  in  any  other  way  "dissociated" 
from  its  fellow  (Chapter  XII).  There  is,  therefore,  at  this  stage, 
'''latent"  or  "  super  able"  squint  in  distant  vision,  combined  with 
"insuperable"  squint  in  near  vision.  At  a  later  stage  the  squint 
becomes,  even  in  distant  vision,  insuperable,  and  thus  what  is 
called  a  " permanent  clement''1  is  by  degrees  developed  in  addition 


Strabism  us  119 

to  the  "  variable  element,"  which  is  added  to  it  whenever  accommo- 
dation is  active. 

Treatment  of  Accommodative  Squint. — Tne  treatment  of 
accommodative  squint  lies  evidently  in  the  correction  of  refrac- 
tion. Less  accommodation  is  then  called  for  and,  therefore,  Itss 
associated  convergence.  The  cure  takes  place  completely  and  at 
once  if  the  squint  be  in  its  early  periodic  stage  ;  but  since  at  the 
time  when  we  first  see  a  squint  the  hypermetropia  has  generally 
become  much  less  than  when  the  squint  began,  owing  to  the 
development  of  the  eye,  the  diminution  of  accommodation  by  the 
spectacles  is  much  less  than  the  excess  of  accommodation  which 
originally  brought  about  the  habit.  The  correction  of  refraction 
does  not  always  cure  even  an  accommodative  squint  at  once,  but 
lessens  it  by  degrees.  Occlusion  of  the  fixing  eye  for  a  considerable 
time,  to  improve  the  working  powers  and  the  visual  acuity  of  the 
habitually-squinting  one,  is  a  good  adjunct. 

Another  treatment  for  accommodative  squint  is  the  instillation  of 
pilocarpine  drops,  which  make  the  ciliary  muscle  respond  more  readily 
to  impulses,  thus  lessening  the  effort  of  accommodation  and  with  it  the 
"  associated  convergence,"  but  it  seems  to  me  to  be  only  palliative. 

Convergent  Squint  Without  Hypermetropia. — A  fair  proportion 

of  convergent  squints  are  found  to  exist  without  hypermetropia  or 
hypermetropic  astigmatism.  Some  of  these  may  possibly,  though 
not  very  probably,  be  due,  as  Buffon  believed,  to  imperfect  visual 
acuity  of  one  eye  leading  to  voluntary  squinting  in  order  to  get  rid 
of  the  disturbing  effect  of  a  blurred  image.  A  far  likelier  history 
in  many  cases  is  that  hypermetropia  existed  at  an  early  age,  which 
has  since  disappeared.  In  others  the  want  of  balance  seems 
inherent  in  the  musculature,  while  yet  others  may  have  had  at 
an  earlier  date  some  paresis  either  of  the  ciliary  muscles  or,  and 
this  is  far  more  common,  of  one  or  both  of  the  external  recti  at 
birth.  In  another  important  class  the  patient  squints  because  there 
is  congenital  deficiency  of  the  fusion  faculty,  just  as  there  is 
deficiency  of  another  kind  in  color  blindness,  and  the  cure  of  the 
one  is  as  hopeless  as  of  the  other.  Any  inequality  in  the  visual 
acuity  of  the  two  eyes  lessens  the  value  of  binocular  vision,  so  that 
the  more  ametropic  eye  is  readily  relinquished  if  by  so  doing  less 
accommodative  effort  is  required.  The  squinting  eye  is  generally 
more  astigmatic  than  the  other,  but  not  always,  for  sometimes  an 
astigmatic  eye  is  the  fixing  one,  while  its  much  more  hypermetropic 


1 20  Tests  and  Studies  of  the  Ocular  Muscles 

fellow  squints.  It  is  then  simply  a  question  of  choice  between 
superior  visual  acuity  or  minimum  effort,  for  of  those  which  have 
astigmatism  in  one  eye  and  higher  hypermetropia  in  the  other, 
some  prefer  distinct  vision  with  a  great  effort  and  use  the  hyperme- 
tropic  eye,  while  others  prefer  less  distinct  vision  with  less  effort 
and  use  the  astigmatic  eye. 

Corneal  Nebulae,  though  they  do  not  cause  squint,  predispose 
to  it  by  lessening  the  value  of  binocular  vision,  and  thus  favor  the 
surrender  of  the  eye  if,  by  that  means,  accommodation  is  facilitated 
or  the  image  freed  from  haze. 

Congenital  Amblyopia,  from  imperfect  development  somewhere, 
probably  plays  an  important  part  in  many,  if  not  most  cases  of  squint 
and  is  to  be  distinguished  from  that  amblyopia  which,  being  simply 
due  to  disuse  and  to  habitual  mental  suppression  of  the  pictures  in 
one  eye,  is  called  amblyopia  ex  anopsia. 

In  nearly  every  case  of  the  ordinary  convergent  squint,  no 
matter  how  amblyopic  the  squinting  eye  may  be,  its  fundus  appears 
perfectly  normal  and  the  macula  tantalizingly  perfect. 

The  element  of  the  amblyopia  which  is  due  to  disuse  can,  I 
think,  to  some  extent  be  distinguished  from  the  congenital  element 
by  a  considerable  difference  in  the  visual  acuity  of  the  outer  and 
the  inner  halves  of  the  retina,  so  that  if  both  of  the  surgeon's  hands 
be  held  up  simultaneously,  one  on  one  side  and  the  other  on  the 
other  side,  while  the  patient  looks  straight  forwards,  the  movements 
of  the  outer  hand  appear  much  more  vivid  to  the  patient  than  those 
of  the  inner,  for  the  probable  reason  that  the  inner  half  of  the  retina, 
since  it  looks  outwards,  has  been  less  disused  than  the  outer  half. 

The  same  "  ex  anopsia"  element  is,  of  course,  still  more  clearly 
demonstrated  by  the  rapid,  though  generally  only  partial,  recovery 
of  visual  acuity  which  attends  continuous  occlusion  of  the  better  eye. 
Even  a  few  days  makes  a  difference,  and  Javal  has  pointed  out  that 
if  the  occlusion  be  long  continued,  improvement  takes  place  some- 
times by  sudden  accessions,  since  the  eye  at  first  is  not  only  wanting 
in  acuity  but  is  awkward  in  seeing,  like  a  raw  recruit,  and  this  takes 
prolonged  practice  to  remedy,  and  is  sometimes  overcome  suddenly, 
as  in  learning  to  swim.  Javal  lays  great  stress  on  imposing- monocu- 
lar vision  in  the  treatment  of  squint,  without  any  intermittence,  so 
that  if  on  special  occasions  it  is  desired  to  permit  the  use  of  the  better 
eye,  the  louchette  should  be  transferred  for  the  time  being  to  the 
squinting  eye. 


Strabismus  121 

Development  of  the  Fusion  Faculty. — The  normal  development 
of  the  fusion  sense  has  been  made  the  subject  of  special  study  by 
Claud  Worth.  He  finds,  as  others  have  done,  that  from  the 
earliest  hours  after  birth  the  pupillary  light-reflex  is  present. 
Indeed,  the  interesting  fact  was  demonstrated  long  ago  that  both 
pupils  respond  to  light  incident  on  one  eye  only,  and  that  the 
reflex  closure  of  the  lids  upon  the  sudden  stimulus  of  light  is 
obtainable  also,  though  the  conscious  perception  of  objects,  so  far 
as  this  can  be  tested  by  closure  of  the  lids  when  an  object  suddenly 
approaches  the  eye,  is  absent  during  the  first  few  weeks.*  Volun- 
tary convergence,  as  in  watching  the  approach  of  an  object  towards 
the  face,  appears  about  the  third  month. 

Worth  has  shown  that  the  preponderance  of  the  macular 
region  exists  at  birth,  since  light  suddenly  thrown  into  an  eye  by  an 
ophthalmoscope  makes  the  eye  immediately  fix  the  mirror,  but 
only  for  a  moment.  The  duration  of  this  monocular  fixation 
increases  during  the  first  few  weeks  and  becomes  binocular  at 
about  the  fifth  or  sixth,  though  still  somewhat  uncertainly  so. 
During  the  last  half  of  the  first  year  of  life,  Worth  has  convinced 
himself  by  prism  experiments  that  true  binocular  vision  has  been 
obtained,  and  that  towards  the  end  of  that  period  the  eyes  will  make 
a  considerable  effort  in  the  interest  of  binocular  vision.  From  the 
results  of  fusion  training  in  the  case  of  squinters,  he  concludes  that 
the  fusion  faculty  is  fully  developed  before  the  end  of  the  sixth  year. 

Defect  versus  Neglect  of  the  Fusion  Faculty. — Congenital 
deficiency  of  the  "desire  for  single  vision"  is  comparatively 
rare.  It  doubtless  accounts  for  the  class  of  alternating  squints, 
with  slight  refractive  error  if  any,  in  which  Javal  pronounced  all 
efforts  to  draw  out  the  faculty  absolutely  hopeless. 

Worth  describes  these  as  ' '  essentially  ' '  alternating  squints, 
and  contrasts  them  with  the  "  accidentally  "  alternating,  which  only 
differ  from  monolateral  squints  in  having  approximately  the  same 
refraction  in  each  eye.  Of  all  constant  squints,  he  finds  fifteen  per 
cent,  are  alternating. 

There  is  reason  to  believe  that  what  is  largely  attributed  to 
congenital  deficiency  of  the  fusion  faculty  is  very  frequently  due 
rather  to  neglected  training  of  that  faculty  in  the  early  years  of 
life  ;  not  because  of  any  fault  in  its  mechanism,  but  because  the 
inferiority  of  one  eye  to  the  other  (which  may  be  transient,  as  are 

*  Preyer,  1884,  quoted  by  Priestley  Smith. 


122  Tests  and  Studies  of  the  Ocular  Muscles 

early  nebulae,  retinal  hemorrhages,  etc. ,  or  permanent,  as  in 
astigmatism  or  anisometropia),  so  much  reduces  the  value  of 
binocular  vision  that  it  is  surrendered  more  readily  in  favor  of 
any  greater  advantage,  as,  for  example,  that  obtained  by  squint- 
ing in  a  hypermetrope.  whose  accommodation  without  it  is  affected 
with  difficulty.  In  the  absence  of  any  such  advantage,  binocular 
vision  is  generally  retained,  even  when  one  eye  is  highly  astigmatic. 
The  love- of  stereoscopic  vision  is,  however,  undoubtedly  more 
intense  in  some  individuals  than  in  others. 

The  easy  success  obtained  by  Worth  in  training  the  fusion 
faculty  in  so  large  a  proportion  of  young  squinters  appears  to  show 
it  had  suffered  from  neglect  more  than  from  anything  else,  and  it 
may  be  noticed  that  squinters  are,  as  a  class,  apt  to  be  naturally 
unobservant. 

Suppression  Of  the  False  Image. — The  longer  a  squint  lasts, 
as  we  have  seen,  the  more  difficult  it  becomes  to  elicit  diplopia, 
because  the  mental  habit  of  suppressing — i.  e.,  of  disregarding — 
one  image,  becomes  confirmed,  and,  in  addition  to  this,  the  longer 
diplopia  is  absent  the  more  difficult  it  becomes  to  re-awaken  fusion 
reflexes. 

Depth  of  the  Suppression. — In  cases  of  suppressed  diplopia  it 
devolves  on  the  surgeon  to  ascertain  the  depth  of  the  suppression  ; 
in  other  words,  whether  diplopia,  though  absent  in  general,  can  be 
artificially  elicited  with  ease  or  with  difficulty. 

(#)  If  with  ease,  a  colored  glass  held  before  the  working  eye 
will  restore  it  when  a  flame  is  looked  at. 

(£)  Failing  that,  a  prism,  edge  up  or  down,  will  be  more 
likely  to  succeed,  by  throwing  the  image  of  the  flame  upon  an 
unusual  part  of  the  retina. 

(r)  Last  of  all,  when  other  means  fail,  the  rod  test,  made  of 
red  glass,  often  succeeds,  if  held  before  the  working  eye,  and 
especially  if  its  effect  is  heightened  by  a  black  velvet  screen  placed 
behind  the  source  of  light  for  "contrast.  Sometimes  a  blue  or  green 
glass  held  before  the  squinting  eye  assists. 

Nature  Of  Suppression  Of  Vision. — Nearly  every  human  faculty 
can  be  quickened  by  concentration  of  attention  upon  it  and  dulled 
by  withdrawal  of  attention.  That  this  is  true  in  the  domain  of 
fusion  I  have  shown  by  a  simple  experiment  with  the  visual  camera, 
described  elsewhere.  In  recently-squinting  eyes,  two  different 
objects  throw  their  pictures  on  the  two  maculae,  but  whichever 


Strabismus  123 

object  engages  attention  for  the  moment  extinguishes  the  mental 
perception  of  the  other.  Indeed,  the  only  object  whose  mental 
appeal  is  effectual,  is  the  false  image  of  the  object  under  attention, 
produced  by  its  picture,  which  falls  on  an  eccentric  part  of  the 
retina  of  the  squinting  eye.  The  same  mental  process  which 
obliterates  the  macular  picture  of  the  squinting  eye  can  in  time 
extend  itself  to  the  false  image  as  well,  and  always  does  so  in 
young  squinters  with  disastrous  effect,  for  the  vision  of  an  eye  thus 
repudiated  becomes  rapidly  impaired  (amblyopia  ex  anopsia).  As 
Priestley  Smith  has  well  put  it,  for  the  squinting  eye,  the  advice 
not  seldom  given,  "to  wait  and  see,"  too  often  means  waiting 
and  not  seeing. 

That  part,  however,  of  the  retina  of  the  squinting  eye  which 
answers  to  the  extreme  temporal  portion  of  the  field  in  a  squint  of 
low  degree,  is  still  of  value  in  the  monocular  perception  of  objects 
which  are  hidden  from  the  other  eye  by  the  root  of  the  nose. 

The  retention  ot  vision  to  a  physiological  amount  in  this 
extreme  portion  of  the  field,  as  compared  with  its  defect  in  the 
opposite  (nasal)  portion  of  the  field,  constitutes  a  point  of  differ- 
ence between  amblyopia  ex  anopsia  and  congenital  monocular 
amblyopia.  This  latter  is  undoubtedly  rare,  and  when  it  exists  is 
probably  due  to  a  defect  of  some  cerebral  cells  rather  than  to  the 
retina  itself,  for  congenital  defects  of  the  eyeballs  are  nearly  always 
bi-lateral,  as  witness  high  hypermetropia,  astigmatism,  lamellar 
cataract,  coloboma,  iridis,  etc. 

Imperfect  Central  Fixation. — When  central  fixation  is  deficient 
from  birth  in  both  eves,  it  nearly  always  causes  nystagmus,  though 
not  necessarily  if  only  one  eye  be  defective.  Probably  most  cases 
of  imperfect  central  fixation  are  acquired  rather  than  congenital 
and,  according  to  Javal,  can  even  be  recovered  by  exercise  pro- 
longed for  years  by  intelligent  subjects,  and  absolutely  free  as 
regards  their  time,  though,  as  he  says  truly,  the  advantage  gained 
is  out  of  all  proportion  to  the  necessary  pains. 

Worth  has  never  seen  "lost  fixation"  in  any  case  of  squint 
first  appearing  after  six  years  of  age.  The  central  region  of  the 
retina  may  suffer  so  much  from  neglect  as  no  longer  to  be  able  to 
count  fingers,  and  Worth  even  states  that  it  may  go  so  far  as  to 
have  only  bare  perception  of  light  within  an  area  extending  25°  to 
30°  from  the  center  of  the  field.  In  congenital  amblyopia  without 
squint,  on  the  other  hand,  he  has  never  found  the  central  vision 


124  Tests  and  Studies  of  the  Ocular  Muscles 

lower  than  /ff  ;  probably  for  the  simple  reason  that  had  it  been 
lower  the  eye  would  have  squinted,  from  binocular  vision  being  of 
so  little  value.  The  earlier  squint  commences,  the  more  rapid  is 
the  progress  of  the  blindness,  provided  it  be  monolateral,  and 
Worth  states  that  at  the  age  of  six  or  eight  months  the  power  of 
central  fixation  is  often  lost  within  eight  or  ten  weeks.  When 
a  squinting  eye  has  lost  its  fixation  power  it  either  wanders 
indefinitely,  when  the  fixing  eye  is  covered,  or  tries  to  fix  with 
some  part  of  the  retina  around  the  fixation  point  ;  or,  as  a  third 
alternative,  it  may  squint  still  farther  inwards  to  use  the  temporal 
part  of  its  field,  which  has  still  retained  the  exercise  of  its  functions 
(eccentric  fixation).  The  highest  vision  possessed  by  such  eyes  is 
to  count  fingers  at  3  or  4  meters  (Asher),  and  Alfred  Graefe  says 
that  often  greater  visual  acuteness  is  obtained  when  the  test  objects 
are  held  in  a  line  with  the  macula  of  the  deviated  eye,  in  spite  of 
the  fact  that  from  sheer  want  of  habit  the  eye  does  not  move  so  as 
to  use  its  macula. 

Its  Diagnosis. — Defective  central  fixation  is  easily  diagnosed  by 
making  the  patient  cover  his  good  eye  and  try  to  fix  the  sight-hole  of 
the  ophthalmoscopic  mirror  with  the  amblyopic  eye  (Priestley  Smith). 
The  corneal  reflexion,  instead  of  occupying  its  steady  and  proper  posi- 
tion, will  appear  to  wander  about.  As  akin  to  this  defect  Javal  notes 
a  certain  number  of  cases  in  which  there  is  a  trembling  of  the  image 
seen  by  the  defective  eye,  even  after  the  power  of  simultaneous 
vision  by  the  two  eyes  has  been  restored,  just  as  a  weak  hand 
trembles  more  than  a  strong  one.  I  have  noticed  this  too,  and 
it  is  not  infrequent. 

Newly- Acquired  Field  of  Fixation  (Perverse  Projection,  or 
Strabismus  Incongruus,  of  Graefe). — It  occasionally  happens  that 
in  squints  of  unusually  fixed  amount,  which  began  in  early  life,  the 
squinting  eye  has  so  far  accommodated  itself  to  its  new  conditions 
as  to  project  objects  in  accordance  with  the  working  eye,  so  that  a 
kind  of  second-rate  binocular  vision  is  retained.  On  putting  such 
an  eye  straight  by  operation,  crossed  diplopia  of  high  degree  imme- 
diately appears,  which  fades  away  in  time.  It  is  not,  of  course,  the 
eye  itself  which  projects,  but  its  cerebral  center ;  so  that  the 
name,  "false  macula,"  often  given  to  this  condition,  is  a  fallacious 
one.  It  is  doubly  fallacious,  since  it  is  not  a  macula  that  is  called 
into  being,  but  a  new  field.  It  has  been  wrongly  described  as  a  small 
part  of  the  retina  which  has  retained  its  function  in  virtue  of  receiv- 


Strabism  us  125 

ing  companion  images  to  those  received  by  the  macula  of  the  best 
eye.  Were  this  view  correct,  there  would  be  no  post-operative 
diplopia,  for  diplopia  means  two  images  of  the  same  object,  and 
the  supposed  solitary  functionating  spot  of  the  retina  cannot,  when 
displaced  by  operation,  receive  a  second  image  from  the  same  object 
as  the  good  macula. 

The  diplopia  observable  is  that  of  images  received  upon  the 
two  true  maculae,  but  the  whole  field  of  the  squinting  eye  having 
been  cerebrally  displaced  for  many  years,  its  macular  impressions 
share  the  displacement  as  much  as  all  other  parts  of  its  retina.  In 
rare  cases  objects  of  similar  appearance  placed  in  line  with  the  two 
foveae  may  be  seen  close  together,  as  well  as  in  the  form  of  crossed 
images  far  apart.  It  is  evident,  therefore,  that  in  consequence  of 
the  squint,  the  faulty  eye  has  acquired  a  new  projection  without 
entirely  forgetting  the  old.  Javal's  view  is  that  there  may  have 
been  fusion  of  the  fields  of  the  two  eyes,  with  mental  suppression 
in  the  case  of  each  of  the  part  which  corresponds  to  the  field 
employed  by  the  other,  since  it  is  generally  not  until  after  operation 
in  these  cases  that  there  is  any  complaint  of  spontaneous  diplopia 
at  all,  and  it  is  sometimes  even  difficult  to  elicit  it  before  operation 
by  red  glass  before  one  eye  and  a  candle. 

Strabismus  Convergens  Myopicus. — In  myopia  of  not  very 
high  degree,  and  in  which  the  value  of  the  two  eyes  is  too  equal 
to  make  it  seem  desirable  to  cheir  possessors  to  surrender  either,  a 
strong  effort  of  convergence  (relatively  to  accommodation)  has  to 
be  made  in  near  vision,  since  the  converging  innervation  is  so 
unsupported  by  any  effort  of  accommodation,  and  the  difficulty 
arises  from  having  to  strongly  assert  one,  and  restrain  the  other,  of 
two  cerebrally  associated  innervations.  This  relatively  strong  con- 
verging activity,  exerted  for  long  periods  at  a  time  by  those  who 
are  engaged  in  reading  or  near  work,  cannot  always  at  once  be 
easily  surrendered  when  distant  objects  are  looked  at,  and  thus 
esophoria  in  distant  vision  becomes  developed  in  consequence, 
gradually  increasing  as  its  cause  continues  till  homonymous 
diplopia  threatens,  then  appears,  persists  and  increases.  In 
near  vision  the  diplopia  is  less,  and  may  even  give  place 
to  slight  exophoria.  A  certain  proportion  of  these  cases,  if  con- 
cave lenses  and  prisms  do  not  relieve  them,  are  grateful  for 
operation,  if  care  be  taken  not  to  create  insufficiency  of  conver- 
gence in  reading. 


126  Tests  and  Studies  of  the  Ocular  Muscles 

Deficient  Abduction  of  the  Squinting  Eye  is  found  not  only  in 
cases  of  paralyses  of  the  sixth  nerve,  but  also  in  ordinary  concom- 
itant convergent  squint  under  certain  circumstances,  though  as  a 
rule  the  restriction  in  outward  movement  is  considerably  less  than 
the  amount  of  squint. 

When  the  restriction  is  very  marked,  it  is  natural  to  suppose^ 
the  primary  cause  to  have  been  an  affection  of  the  sixth  nerve,  and 
if  there  be  any  corresponding  want  of  concomitancy,  the  supposi- 
tion is,  without  doubt  correct  :  it  may  even  be  correct  when  con- 
comitancy exists  over  the  whole  motor  field  up  to  the  area  of 
restriction,  for  though  the  concomitancy  shows  that  the  nerve  has 
recovered  its  power,  its  paralysis  may  have  been  the  original  cause. 

But  in  many  cases  the  restriction  is  simply  due  to  want  of  habit, 
and  has  no  pathological  meaning.  It  is  when  the  squinting  eye  is 
highly  amblyopic  in  all  parts  of  its  field  of  vision  and  when,  there- 
fore, the  amblyopia  existed  from  infancy  and  preceded  the  squint, 
that  this  explanation  is  most  probable,  there  being  then  no  object 
gained  in  turning  the  eye  outwards.  Secondarily,  perhaps,  the 
rectus  may  be  weak  for  want  of  use  ;  but  this  corrects  itself,  I 
believe,  in  time  if  the  eye  is  brought  into  use. 

If  the  deficient  abduction  be  due  to  defect  of  innervation, 
instead  of  tenotomizing  the  internal  rectus  of  the  squinting  eye  that 
of  the  sound  one  should  be  divided,  so  as  to  call  the  defective 
innervation  into  play.  It  is  sometimes  better,  however,  to  advance 
the  external  rectus  of  the  squinting  eye. 

When  restricted  abduction  is  really  due  to  an  evident  defect  of 
the  sixth  nerve,  advancement  of  the  external  rectus  is  the  only 
justifiable  operation,  reinforced,  if  needed,  by  tenotomy  of  the 
internus  of  the  same  eye. 

A  very  useful  adjunct  to  tenotomy  I  find  to  be  stretching  the 
soft  cicatrix  if  a  greater  effect  is  desired.  It  can  be  done  daily  for 
several  days  after  the  operation.  The  way  I  proceed  is  as  follows  : 
After  pressing  a  small  plug  of  cotton  wool  dipped  in  cocaine  solution 
and  held  by  fixation  forceps,  against  the  conjunctiva  close  to  the 
outer  margin  of  the  cornea,  the  conjunctiva  is  tightly  gripped  and 
the  eye  drawn  slowly  and  steadily  out  while  the  patient  fixes  with 
his  other  eye  an  object  on  the  other  side  of  the  room.  The  eye  is 
then  held  in  this  position  of  divergence  for  about  a  minute,  during 
which  it  yields  a  little  more.  The  idea  was  suggested  by  the 
so-called  "mechanical"  treatment  of  squint  by  stretching  the 


Strabismus  127 

muscle  without  operation,  of  which,  however,  I  have.no  experience, 
as  it  does  not  sound  a  practical  idea. 

Divergent  Strabismus. — The  eyes  when  free  from  active  in  ner- 
vation tend  to  settle  down  into  divergence.  Healthy  eyes  diverge 
under  chloroform  and  during  sleep,  and  even  the  so-called  "per- 
manent" element  of  a  convergent  strabismus  may  completely  dis- 
appear under  the  chloroform.  This  seems  to  confirm  Bonders' 
view,  that  while  the  development  of  convergent  squint  is  an  active, 
that  of  divergent  squint  is  a  passive  process.* 

With  a  few,  sometimes  rather  inexplicable  exceptions,  blind 
eyes  in  emmetropic  individuals  tend  to  diverge,  especially  in  adults. 
The  exceptions  consist  of  those  who  had  esophoria  previously, 
either  from  weakness  of  the  external  recti,  from  anatomical 
anomalies  of  the  ocular  muscles,  from  ciliary  paresis,  or  from 
habitual  over-tonicity  of  the  converging  innervation. 

In  Myopia. — While  there  may  be  some  truth  in  the  statement 
that  the  elongated  shape  of  myopic  eyes  opposes  an  obstacle  to 
convergence,  the  want  of  support  to  convergence  due  to  the 
absence  of  accommodative  effort,  is  no  doubt  the  chief  cause  of 
that  myopic  exophoria  in  near  vision  which  often  exists  to  so  high  a 
degree,  even  in  eyes  of  equal  refraction. 

The  higher  the  myopia  the  greater  is  the  effort  of  convergence 
in  reading  or  fine  work,  and  this  effort  being  unsupported  by  its 
companion  innervation  may  cause  sufficient  fatigue  of  the  converg- 
ing center  to  allow  at  times  one  eye  to  deviate.  If  it  does  so  at 
all,  it  does  so  considerably,  so  as  to  minimize  the  trouble  occasioned 
by  the  diplopia.  When  once  the  habit  has  commenced,  it  gains  in 
frequency  and  may  lead  to  a  permanent  squint  in  both  near  and 
distant  vision.  The  treatment  in  the  early  stages  is  evidently  to 
correct  the  myopia  in  whole  for  young  people  or  in  part  for  older 
ones,  so  as  to  lessen  the  convergence  and  introduce  at  the  same 
time  an  act  of  accommodation. 

With  Anisometropia. — When  any  considerable  difference  exists 
between  the  value  of  the  two  eyes,  the  effort  of  convergence  may 
be  greater  than  the  usefulness  of  the  worst  eye,  which  the  patient, 
therefore,  at  times  allows  to  deviate  outwards  by  discontinuing  the 
converging  effort,  especially  if  he  finds  that  by  so  doing  accommo- 
dation can  be  more  completely  relaxed,  or  if  the  print,  as  seen  by 

*  Ponders'  antithesis  is  :  "  Hypermetropia  causes  accommodative  asthenopia,  to  be  actively 
overcome  by  strabismus  convergciis.  Myopia  leads  to  muscular  astheuopia,  passively  yielding 
to  strabismus  divergeus. 


i  j.s  '/;  .v/j-  and  Studies  of  the  Ocular  Muscles 

one  eye,  is  more  distinct  than  when  seen  by  both.  As  Javal  points 
out,  binocular  vision  has  less  value  for  reading  than  for  most  other 
acts  of  vision,  since  in  a  page  of  print  there  is  no  "  third  dimension. 
For  this  reason  habitual  latent  divergence  (or  "suppressed" 
squint)  is  all  the  more  apt  to  give  place  to  "  manifest  "  squint  on 
occasions  which  becomes  more  and  more  frequent,  until  it  persists 
altogether,  and  involves  distant  vision  as  well. 

Age  Relation. — While,  therefore,  convergent  squint  is  of  infan- 
tile origin,  the  divergent  variety  commences  towards  adult  life.  This 
is  due  to  the  fact  that  children  are  so  rarely  myopic,  and  to  the  ado- 
lescent increase  of  myopia.  The  practice  of  steady  reading,  too, 
increases  in  the  years  of  adolescence.  And  again,  as  mentioned 
elsewhere,  the  tonic  activity  and  excitability  of  the  converging 
center  seem  to  lessen  with  age,  which  also  favors  divergence. 

Refractive  After-Treatmerit  for  Squint. — Over-correction  of 
cotwergent  squint  is  generally  advantageously  followed  by  either 
no  correction  or  a  considerable  under-correction  of  any  existing 
hypermetropia,  and  it  is  sometimes,  I  think,  a  good  plan  to  make 
young  emmetropes  wear  even  weak  concave  lenses. 

On  the  other  hand,  operative  under-correction  of  a  convergent 
squint  clearly  indicates  full  correction  of  the  hypermetropia  in  dis- 
tant vision  and  perhaps  even  an  over-correction  in  near  vision,  in 
order  to  gradually  supplement  the  effect  of  the  operation. 

Operative  over-correction  of  myopic  convergent  squint  should 
be  followed  by  constant  use  of  the  full  refractive  correction. 

Over-correction  of  divergent  squint,  if  it  remains,  interposes  a 
serious  difficulty  in  the  restoration  of  binocular  vision,  because  a 
diverging  effort  is  much  more  difficult  to  make  in  the  interest  ot 
fusion  than  a  converging  effort.  Stronger  plus  lenses,  or  weaker 
minus  ones,  are  therefore  indicated. 

Under-correction  of  divergent  squint  indicates  weaker  plus  or 
stronger  minus  lenses. 

The  above  rules,  it  need  hardly  be  said,  apply  only  to  the 
"post-operative"  treatment  of  squint,  i.  e.,  after  everything  has 
been  done  that  is  indicated  in  an  operative  way  ;  and  they  are  only 
intended  to  give  the  ' '  fine  adjustment ' '  at  last.  For  every  operator 
knows  that  after  putting  a  squint  straight  to  the  perfect  satisfaction 
oi  the  patient,  the  effect  is  generally  either  a  trifle  less  or  a  trifle 
more  than  his  own  ideal.  It  is  true  that  when  binocular  vision 
is  restored  the  defect  is  entirely  covered,  and  exists  only  as  a 


Strabismus  129 

"  heterophoria"  ;  but  there  it  is,  all  the  same,  and  a  slight  modifi- 
cation of  the  optical  correction,  when  the  patient  is  (as  usual) 
young  enough  not  to  mind  it,  can  either  increase  or  lessen  the 
activity  of  the  tonic  convergence,  so  as  to  correct  by  degrees  even 
that  heterophoria.  Thus,  with  residual  "esophoria,"  a  low  hyper- 
metrope  might  dispense  with  glasses  altogether  for  a  time,  and  a  high 
hypermetrope  wear  a  somewhat  weaker  pair.  There  is  no  need  to 
make  the  modification  a  great  one,  so  long  as  it  is  in  the  right 
direction.  Nor,  of  course,  is  optical  adjustment  intended  to  take 
the  place  of  fine  second  operative  adjustment  in  the  event  of  the 
first  operation  being  markedly  insufficient.  It  is  essentially  "  post- 
operative." 

In  long-standing  convergent  squints,  where  the  restoration  of 
binocular  vision  is  impossible,  perfect  straightness  is  to  be  regarded 
as  undesirable,  since  it  leaves  no  margin  for  the  natural  diminution 
of  converging  activity,  which  goes  on  year  after  year.  In  such  a 
case,  therefore,  a  slight  reduction  should  be  made  from  the  full 
hypermetropic  correction,  unless  the  patient  be  willing  for  another 
slight  operation  a  year  or  two  later  on.  At  least  three  degrees  of 
residual  convergence  should  be  aimed  at  in  cases  where  binocular 
vision  is  irrecoverable,  and,  thanks  to  the  high  angle  gamma,  which 
generally  exists  in  hypermetropes,  the  eyes  do  not  betray  a  residuum 
of  even  five  degrees  to  ordinary  observers. 

Author's  Combined  Bar-Reader  and  Squint  Stereoscope.— This 
instrument  is  intended  to  be  placed  in  the  hands  of  a  patient.  A 
saw-cut  is  made  round  the  lenses  of  a  Holmes  stereoscope  and  two 
hinges  put  on,  so  that  the  lenses  can  fall  down  out  of  the  way  when 
the  instrument  is  used  for  bar-reading.  Two  pieces  of  talc  or 
mica  are  pivoted  into  saw-cuts  and  are  scratched  by  gentle  grada- 
tion more  and  more  from  their  inner  or  lower  edge  to  their  outer 
edge.  By  gradually  lowering  one  of  these  before  the  non-squinting 
eye,  its  vision  is  gradually  lessened  till  the  image  that  belongs  to 
the  squinting  eye  springs  into  view. 

Another  plan  is  to  begin  with  the  talc  shutter  down,  and  while 
the  squinting  eye  is  examining  some  near  or  distant  object,  to 
gradually  allow  the  good  eye  to  be  uncovered,  while  still  trying  to 
keep  the  squinting  eye  in  use.  In  this  way  a  monolateral  squint 
can  be  readily  trained  to  be  alternating.  For  bar-reading,  the  oval 
containing  the  lenses  should  be  turned  down  on  its  hinges.  Another 
new  feature  of  the  instrument  is  that  it  is  provided  with  an 


130  Tests  and  Studies  of  the  Ocular  Muscles 

extensible  median  partition,  which  stretches  from  the  middle  of  the 
card  to  between  the  two  lenses.* 

Natural  Cure  and  Natural  Increase.— Convergent  squints 
become  gradually  less  as  years  go  by.  In  the  case  of  young 
children  this  is  partially  due  to  the  physiological  growth  of  the 
eye  causing  diminution  of  their  hypermetropia,  its  exciting  cause. 
But,  besides  this,  the  converging  center  appears  to  lose  its  excita- 
bility with  age,  and  even  squints  in  which  no  hypermetropia  at  all  is 
to  be  found,  tend  to  get  less  in  course  of  time.  Divergent  squints, 
however,  generally  get  worse  as  years  roll  on. 

Treatment  of  Fixed  Convergent  Squint. — We  have  seen  that 
convergent  squints  tend  in  the  course  of  years  to  undergo  a 
natural  cure.  The  correction  of  any  hypermetropia,  or  hyper- 
metropic  astigmatism,  of  course,  expedites  this  natural  cure  and 
should  always  have  a  good  trial,  except  in  squints  of  very  high  degree, 
in  which  the  patient  might  have  to  wait  many  years  and  thus  lose  the 
likelihood  of  regaining  binocular  vision.  In  such  cases  a  good  plan  is 
to  wear  glasses  for  six  months,  and  if  the  squint  is  found,  by  measure- 
ment before  and  after,  not  to  have  notably  decreased,  an  operation 
should  be  performed,  so  as  to  lose  no  more  time.  In  the  mean  time, 
steps  should  be  taken  to  lessen  amblyopia  in  the  squinting  eye,  by 
occlusion  of  the  good  one,  and  to  recover  any  lost  faculties  by  training. 

Since,  however,  it  is  easier  to  lose  faculties  than  to  regain 
them,  it  is  important,  as  emphasized  by  Javal  and  Priestley  Smith, 
to  commence  the  treatment  of  squint  at  as  early  an  age  as  possible. 
Refraction  should  be  corrected,  or,  if  that  be  impracticable,  atropine 
may  be  used  continuously  for  a  time  in  the  best  eye  if  the  squint  be 
unilateral  or  in  both  if  it  be  alternating.  The  effect  of  atropine 
should  be  watched,  so  as  to  discontinue  it  if  it  does  not  markedly 
diminish  the  squint.  Occlusion  of  the  squinting  eye  (Buffon, 
Javal)  is  better  still.  Personally,  I  am  not  much  in  favor  of 
atropine  for  continued  use.  Both  Javal  and  Priestley  Smith  speak 
of  operating  as  early  as  two  years  of  age,  if  necessary.  To  do  this 
we  must  first  feel  confident  about  the  certainty  of  restoring  binocular 
vision,  otherwise  the  eye  will  turn  out  in  later  life.  With  congenital 
squints  we  cannot  have  this  confidence,  and  it  is  wise  to  approach 
them  with  caution  ;  but  when  there  is  a  definite  history  of  the 
squint  having  been  preceded  by  straight  eyes,  with  an  interval 
during  which  it  was  periodic,  and  especially  if  we  can  still  elicit 

*  The  instrument  can  be  obtained  from  E.  Long,  Tangley,  Bournemouth,  England. 


Strabismus  131 

diplopia  or  excite  fusion  by  stereoscopic  devices,  an  operation  can 
be  done  without  fear. 

Recovery  Of  Lost  Faculties. — Javal  has  been  the  chief  pioneer 
in  this  direction.  The  steps  in  cure,  according  to  him,  are  : 

(1)  Restoration    of    the   power    of    simultaneous    vision,    as 
evidenced  by  diplopia,   by  overcoming    the    habit    of   suppressing 
the  image  of  the  faulty  eye.     The  chief  agent  to  this  end  is  the 
permanent  monocular  occlusion. 

(2)  Overcoming  this  diplopia  \>y  fusion.      For  this  the  stereo- 
scope   is    useful,   and   also    exercises  without  the  sterescope,   with 
flames  and    prisms   arranged  to  excite    the    desire    to    see   single. 
For  convergent  squints  of    high  degree,  Javal   uses  Wheatstone's 
stereoscope. 

(3)  The  perception  of  relief  by  suitable  motions  of  the  eyes. 
For  this  the  stereoscope  may  again  be  pressed  into  service.     The 
length  of  time  required  to  re-establish  binocular  vision,  according 
to  his  experience,  is  nearly  equal  to  that  which  has  elapsed  since 
the  squint  began. 

Let  us  look  at  these  in  detail. 

Occlusion. — The  object  of  this  is  to  overcome  the  mental  sup- 
pression of  the  false  image.  Javal' s  idea  is  to  have  either  one  or 
the  other  eye  always  covered,  so  that  more  than  one  image  at  a 
time  is  never  seen,  in  the  hope  that  the  brain  will  forget  how  to 
suppress  the  second  image.  It  is  best  to  cover  the  good  eye,  but, 
as  a  luxury,  the  cap  may  sometimes  be  transferred  to  the  squinting 
one.  Few  oculists  pursue  this  treatment  so  completely  as  Javal 
recommends  ;  occlusion  for  a  few  hours  a  day  is  a  more  common 
prescription  but  not  nearly  so  effective.  To  equalize  the  vision  of 
the  two  eyes  I  sometimes  prescribe  a  deeply-tinted  glass  before  the 
better  one,  with  one  or  two  opaque  bands  across  it. 

Orthoptic  Training. — Javal' s  cartons  contain  numerous  devices 
and  are  of  great  service.  They  are  intended  for  use  with  either  an 
ordinary  stereoscope  or  (if  cut  in  two)  with  a  modification  of  a 
Wheatstone  stereoscope,  arranged  by  Javal  under  the  name  of  a 
"sterescope  a  charniere. "  As  a  preliminary  to  stereoscopes, 
however,  I  prefer  to  use  a  very  simple  device  lately  introduced 
by  Priestley  Smith  and  to  which  he  has  given  the  name  of  "  fusion 
tubes."  They  consist  of  two  short  tubes,  held  together  by  chains, 
for  the  squinter  to  look  through.  Each  is  provided,  at  the  eye  end, 
with  a  convex  lens  whose  focal  length  is  equal  to  the  length  of  the 


1^2  Tests  and  Studies  of  the  Ocular  Muscles 

tubes,  and  the  other  end  is  closed  by  an  opaque  disk  perforated 
with  two  translucent  holes.  The  hole  in  the  center  of  each  disk  is 
white,  while  the  neighboring  hole  is  red  in  one  tube  and  green  in 
the  other.  On  looking  into  these  tubes,  a  squinter  with  binocular 
vision  sees  four  holes,  two  of  which  are  white,  one  red  and  the  other 
green.  By  moving  the  tubes,  the  two  white  holes  can  be  brought 
together  and  fused.  Three  holes  only  are  then  seen,  red,  white  and 
green.  The  red  and  green  holes  act  like  Javal's  "control  marks," 
to  insure  that  true  fusion  exists  and  not  merely  suppression  of 
images  by  one  eye.  By  now  moving  the  tubes  so  as  to  slightly 
separate  the  white  holes  and  thus  making  a  strong  cerebral  effort  to 
fuse  them,  the  eyes  can  be  trained  to  overcome  a  squint. 

The  great  quality  required  is  perseverence,  and  when  I  have  been 
able  to  meet  with  it,  the  fusion  tubes  have  proved  very  successful. 
One  nurse  maid,  for  instance,  with  a  periodic  divergent  strabismus 
of  15°,  restored  her  eyes  to  perfect  orthophoria  in  three  months. 
A  boy  with  more  than  5°  of  vertical  squint  restored  his  eyes  to 
parallelism  within  the  same  time.  Fusion  tubes  mounted  in  a 
more  elaborate  way,  so  as  to  measure  the  squint,  constitute  the 
heteroscope  of  Priestley  Smith.  By  affixing  translucent  gum  paper 
to  the  farther  end  of  the  fusion  tube  before  the  better  eye,  the  holes 
seen  by  that  eye  can  be  darkened.  This  enables  the  other  eye  to 
see  better.  Landolt  has  introduced  a  stereoscope  to  facilitate  this 
plan,  in  which  similar  tubes  are  used,  but  pictures  are  employed 
and  the  farther  end  can  be  darkened  by  an  iris  diaphragm.  I  do 
not  know  if  Tourmaline  plates  have  yet  been  suggested,  but  they 
would  doubtless  act  very  well.  Quite  recently  C.  Worth,  of  Lon- 
don, has  brought  out  an  arrangement  of  tubes  which,  in  principle, 
is  as  if  the  "stere"scope  &  charniere"  were  mounted  in  tubes,  with 
translucent  pictures  mostly  like  Perlia's,  intended  to  be  unequally 
illuminated  by  lamps  placed  opposite  the  two  tubes.  By  lowering 
the  lamp  before  the  good  eye,  the  picture  .before  the  other  eye 
becomes  visible.  The  instrument  promises  to  be  useful  for 
squints  of  higher  degree  than  the  fusion  tubes  can  suit  and  has 
the  advantage  of  permitting  any  number  of  designs.  When 
an  ordinary  stereoscope  is  available,  Perlia's  excellent  pictures 
may  be  used. 

Magnetic  Stereoscope.— This  is  a  new  apparatus  which  I  have 
constructed  and  which  appears  likely  to  be  very  effective.  The 
patient  looks  into  an  ordinary  stereoscope  fitted  with  small  electro- 


Strabismus  133 

magnets  which  move  a  black  feather  at  the  end  of  a  straw,  so  as  to 
cut  off  the  vision  of  either  eye  at  the  will  of  the  surgeon,  who  sits 
in  a  chair  at  any  convenient  distance  and  presses  a  button  on  a 
separate  piece  of  wood  to  occlude  the  right  eye  or  another  button 
to  occlude  the  left.  The  movement  of  the  feather  is  almost  instan- 
taneous. By  interposing  an  interrupting  hammer  in  the  circuit  or 
a  metronome  with  a  wire  across  its  lever,  the  two  ends  of  which 
dip  alternately  into  pools  of  mercury,  the  work  of  the  surgeon  can 
be  done  mechanically.  The  apparatus  can  be  used  in  several  ways. 
One  of  Javal's  cartons  is  placed  in  the  stereoscope,  such  as  an  L 
before  one  eye  and  an  F  before  the  other,  or  a  pictorial  representa- 
tion of  a  stable  before  one  eye  and  a  horse  before  the  other.  Any 
one  of  the  following  plans  can  be  adopted  : 

(a)  By  intermittent  occlusion  of  the  fixing  eye  alone  in  a 
case  of  deep  suppression  of  the  false  image,  the  latter  comes  into 
view.  By  degrees  the  intermission  can  be  made  so  rapid  that  the 
true  image  is  not  lost  at  all,  and  thus  both  images  are  seen 
simultaneously. 

(^)  The  feather  can  be  made  to  occlude  each  eye  in  turn,  at 
first  slowly  and  then  more  rapidly.  This  keeps  both  eyes  "alive," 
as  it  were,  and  incites  them  to  act  more  and  more  simultaneously. 

(r)  With  the  good  eye  occluded,  the  briefest  possible  uncov- 
ering of  it  may  be  made,  before  and  during  which  the  patient  is 
told  to  carefully  watch  the  image  before  the  suppressing  eye  so  as 
not  to  lose  sight  of  it.  The  interval  can  then  be  lengthened  by 
degrees.  The  apparatus  was  suggested  by  some  physiological 
experiments  I  made  with  the  visual  camera  about  seventeen  years 
ago,  but  I  have  only  recently  applied  it  clinically. 

Extension  of  Partially-Preserved  Faculties. — The  slightest 
retention  of  binocular  vision  in  comitant  squints  affords  a  very 
encouraging  factor  in  prognosis  and  squints  can  be  approached  for 
operation  with  far  more  confidence  if  it  exists,  since,  once  restored, 
it  has  a  keeping  power  which  prevents  the  return  of  the  squint  as 
well  as  the  power  of  perfecting  the  straightness  of  the  eye  utterly 
beyond  any  operative  ability.  For  this  reason,  operations  for 
unilateral  strabismus  should  be  preceded  by  at  least  a  month  of 
permanent  occlusion  of  the  better  eye,  if  there  is  any  hope  of 
restoring  binocular  vision. 

In  incomitant  squint,  when  binocular  vision  still  remains  in  no 
matter  how  small  a  corner  of  the  field,  its  extension  by  judicious 


134  Tests  and  Studies  of  the  Ocular  Muscles 

operation  is  feasible,  and  even  without  operation  it  may  be  extended, 
as  Javal  suggests,  by  daily  training  in  which  the  patient  fixes  some 
bright  object  with  great  attention  while  slowly  moving  his  head 
so  as  to  bring  the  vision  of  it  to  the  furthest  limits  of  his  field  of 
single  vision. 

Evidences  Of  Squint. — The  most  decisive  evidences  of  a  squint 
are  diplopia  and  the  appearance  of  a  manifest  deviation  of  one 
eye.  The  diplopia  may,  however,  be  missing,  from  blindness  of 
one  eye  or  from  the  habit  of  mentally  suppressing  the  false  image, 
and  the  deviation  may  be  apparent  rather  than  real.  This  makes 
it  necessary  to  have  tests  at  our  disposal  to  make  sure. 

Exclusion  Test  for  Squint. — As  this  useful  old  test  is  frequently 
spoilt  by  the  student  too  indefinitely  shifting  his  hand  from  one  eye 
to  another,  it  may  be  well  to  describe  it  minutely. 

Direct  the  patient's  attention  to  some  small  and  rather  distant 
but  perfectly  distinct  object,  and  after  ensuring,  by  watching  his 
eyes  for  a  moment  or  two,  that  his  gaze  is  steadily  fixed,  suddenly 
cut  off  the  vision  of  one  eye — say  the  right — by  a  swift  lateral 
movement  of  the  left  hand,  made  from  the  wrist,  with  the  fingers 
extended  and  the  dorsum  towards  the  patient's  eye,  but  without 
touching  any  part  of  his  face.  If  the  left  eye  make  no  corrective 
movement  but  remain  as  immobile  as  ever,  it  is  acquitted  from 
squinting. 

But  the  excluded  eye  is  not  yet  acquitted,  for  it  may  be  the 
squinting  one  ;  therefore,  now,  after  waiting  again  a  moment  or  two 
to  ensure  that  the  patient  is  steadily  fixing,  cover  his  left  eye  sud- 
denly with  the  right  hand.  If  the  right  eye  remain  immobile,  it 
also  is  innocent.  No  squint,  therefore,  exists. 

If,  however,  either  eye  should  make  a  little  inward  ' '  correc- 
tive "  movement  when  its  neighbor  is  covered,  it  must  have  been 
previously  squinting  outwards,  and  if  it  make  a  little  oiitward 
"corrective"  movement,  it.  must  have  been  previously  squinting 
inwards. 

The  test  for  manifest  squint  must  be  distinguished  from  the 
exclusion  test  for  suppressed  squint,  described  later,  in  which  the 
procedure  is  entirely  different. 

Majiifest  squint  .is  that  which  exists  when  both  eyes  are  naked. 

Latent  or  "suppressed"  squint  is  that  which  only  arises  when 
one  eye  is  excluded  from  vision,  or  is  in  some  way  dissociated  from 
its  neighbor. 


Strabismus  135 

Subjective  Screen  Test. — The  following  is  translated  from 
Alfred  Graefe  : 

"Suppose,  for  example,  that,  on  account  of  paralysis  of  any 
muscle  of  the  right  eye,  slight  deviation  of  the  visual  axis  is  present 
for  some  determined  position  of  the  object.  If  we  now  cover  the 
right  eye  during  fixation,  the  image  of  that  eye  will  disappear  and 
that  of  the  left  retain  its  position,  since  the  sound  (left)  eye, 
engaged  in  fixation,  will  continue  undisturbed  therein. 

' '  Let  us,  however,  under  similar  conditions,  cover  the  left  eye  ; 
its  image  will  correspondingly  disappear,  but  simultaneously  the 
still  remaining  image  of  the  right  eye  will  exhibit  a  change  of  posi- 
tion, since  the  right  eye  now  for  the  first  time  directs  itself  for 
fixation  and  has  to  make  a  proportionate  excursion  to  bring  its 
hitherto  eccentrically-placed  retinal  picture  to  the  spot  of  central 
vision."* 

Secondary  Deviation. —  When  we  compel  a  squinting  eye  to 
take  up  fixation  by  placing  a  screen,  such  as  the  hand  or  piece  of 
ground  glass,  before  the  eye  which  naturally  fixes,  the  squint  is 
transferred  from  one  eye  to  the  other,  and  the  deviation  of  the  eye 
behind  the  screen  is  called  the  "secondary  deviation,"  to  distin- 
guish it  from  "primary  deviation,"  which  is  the  deviation  of  the 
squinting  eye  under  ordinary  conditions. 

In  alternating  squint^  we  have  seen  that  the  transference  of 
the  squint  from  one  eye  to  the  other  remains  after  withdrawal  of 
the  screen,  one  eye  being  as  prone  to  squint  as  the  other  and  the 
patient  having  no  preference  as  to  which  he  uses  for  fixation.  With 
alternating  squints,  therefore,  we  cannot  draw  any  distinction 
between  primary  and  secondary  deviations  :  in  these  cases  it  will 
generally  be  found  that  there  is  great  approach  to  equality  in  the 
visual  acuity  of  the  two  eyes. 

In  unilateral  squints,  the  secondary  deviation  gives  place 
again  to  the  primary  as  soon  as  the  screen  is  withdrawn,  the  squint 
being  again  transferred  to  its  original  seat. 

It  is  easy  to  observe  the  relative  amplitude  of  the  two  devia- 
tions, for  the  extent  of  the  secondary  deviation  can  be  watched 
through  a  piece  of  ground  glass  (Javal)  or  even  behind  an  opaque 
screen,  if  the  latter  be  held  obliquely  so  as  to  let  the  eye  be  visible 


*  Alfred  Graefe  ;  "  Motilitatsstfirungen  "  (1858),  p.  21. 

t  There  is  a  large  intermediate  group  of  cases  which  are  nearly  alternating,  the  patient 
being  able  to  fix  with  either  eye  but  preferring  one  above  the  other.  In  these  cases  the 
secondary  deviation  remains  for  some  time  after  withdrawal  of  the  screen. 


1 36  Tests  and  Studies  of  the  Ocular  Muscles 

whik-  yet  it  is  cut  off  from  fixation  ;  or,  better  still,  the  hand  can  be 
withdrawn  instantaneously  long  enough  to  see  the  deviation  without 
giving  it  time  to  disappear. 

When  there  is  the  slightest  paralytic  element  in  the  squint  the 
secondary  deviation  is  infallibly  greater  than  the  primary,  and  the 
more  so  the  more  the  direction  of  fixation  becomes  such  as  to 
require  contraction  of  the  affected  muscle. 

An  extremely  delicate  test,  therefore,  for  paresis  is  to  test  the 
secondary  deviation  at  the  extreme  periphery  of  the  motor  field  in 
the  direction  of  action  of  the  paralyzed  muscle. 

In  comitant  squint  the  primary  and  secondary  deviations  are 
equal  and  the  amplitude  of  the  squint  remains  unchanged  over 
the  whole  motor  field,  except  sometimes  near  its  periphery  from 
mechanical  hindrances  :  this  kind  of  squint  is  due  to  anomaly  of  a 
conjugate  innervation.* 

Persistent  Secondary  Deviation. — It  sometimes  happens  that  in 
a  patient  affected  with  paresis  of  an  ocular  muscle,  the  affected  eye 
has  so  much  the  better  vision  of  the  two  that  he  prefers  to  use  it  for 
habitual  fixation.  In  this  case  he  goes  about  with  a  "persistent 
secondary  deviation  "  of  the  unparalyzed  eye,  which  may  deceive 
a  careless  investigator  and  make  him  blame  the  wrong  eye.  Such 
cases  are  somewhat  rare,  however,  for  the  paralyzed  eye  can  only 
be  used  at  the  cost  of  giddiness,  malprojection  and  unsteadiness,  to 
reduce  which  to  the  minimum  the  patient  goes  about  with  his  head  so 
inclined  as  to  give  the  weak  muscle  as  little  work  to  do  as  possible. 

Reason  Of  Secondary  Deviation.— It  remains  to  explain  why,  in 
paralytic  squint,  the  amplitude  of  the  secondary  deviation  is  greater 
than  that  of  the  primary.  It  is  simply  due  to  the  fact  that  no 
considerable f  impulse  can  travel  to  any  muscle  without  an  equal 
impulse  being  sent  to  its  associated  muscle  in  the  other  eye.  The 
normal  eye  faithfully  responds  to  its  received  impulses  and  moves 
in  exact  proportion  to  their  strength,  but  the  paralyzed  muscle  is 
unable  to  respond  in  the  same  proportion,  if  at  all.  When  the 
squinting  eye,  therefore,  is  compelled  to  take  up  fixation,  half  of 
the  great  effort  required  is  vainly  spent  on  the  weakened  muscle, 
while  the  other  half  produces  a  high  degree  of  movement  in  the  nor- 
rnal^eye — a  degree  which  measures  the  amount  of  effort  put  forth. 

•Cases  of  non-paralytic  squint  I  have  met  with   in  which  the  secondary  deviation  is  dis- 
ly  greater  than  the  primary.    In  others,  especially  with  H.  or  H.  As.,  the  reverse  may  be 
the  case,  from  want  of  visual  effort  in  the  squinting  eye 

'  "considerable,"  because  in  the  interests  "of  fusion  the  eves  can  in  a  small  degree 
icmselves  in  a  way  which  does  not  seem  to  obey  the  laws  of  conjugate  motion. 


Strabismus  137 

Fallacy  from  Anisometropia. — There  is  one  fallacy  to  be  guarded 
against  in  testing  the  secondary  deviation  :  if  one  eye  be  more  hyper- 
metropic  than  the  other,  the  secondary  deviation  may  be  greater  or 
less  than  the  primary,  owing  to  the  greater  amount  of  accommoda- 
tion required  from  one  eye,  producing  a  proportionately  greater 
amount  of  associated  convergence. 

Apparent  Squint. — The  appearance  of  squint,  as  has  been  said, 
may  be  illusory.  Myopic  eyes  frequently  give  the  impression  of  a 


Fig.  45 

Ophthalmoscopic  corneal  reflections  in  Emmetropic  eyes ;    (a)  with  both  eyes  looking  at  the 

center  of  the  mirror  (6)  with  both  eyes  looking  to  the  right,  showing  asymmetry  of  the 

corneal  images  owing  to  the  angle  alpha. 

slight  convergent  squint,  while,  on  the  other  hand,  hypermetropic 
eyes,  though  not  quite  so  deceptively,  often  appear  divergent. 

We  can  settle  any  doubts  in  our  mind  as  to  whether  the  squint 
is  a  real  one,  by  the  "exclusion  test,"  already  described;  or, 
better  still,  by  placing  the  patient  with  his  back  turned  three- 
quarters  towards  the  window  (  or  with  his  head  near  a  flame,  if  the 
room  be  dark),  and  reflecting  the  light  on  to  first  one  eye  and  then 
the  other  from  the  mirror  of  the  ophthalmoscope,  held  about  nine 
inches  from  the  face.  The  observer  should  look  through  the 
aperture  of  the  ophthalmoscopic  mirror  as  if  about  to  examine  the 
patient's  fundus,  and  also  direct  the  patient' s  attention  carefully  to 
the  same  aperture.  A  tiny  circular  reflection  from  the  mirror  will 
now  be  visible  in  each  eye,  as  in  Fig.  45,  about  -^  inch  in  diameter, 
but  smaller  still  the  farther  away  the  mirror  is  held. 

In  emmetropic  eyes  the  reflection  will  appear,  as  in  the  figure, 
slightly  to  the  inner  side  of  the  center  of  each  cornea,  and  if  they 
are  symmetrically  disposed  in  the  two  eyes,,  the  existence  of  squint 
may  be  safely  denied. 


I38  Tests  and  Studies  of  the  Ocular  Muscles 

If  the  deceptive  appearance  has  been  due  to  myopia,  the  reflec- 
tions will  lie  nearer  than  usual  to  the  center  of  each  cornea  ;  but  if  to 
hypermetropia,  they  will  both  appear  displaced  farther  inwards  than 
usual.  The  fuller  treatment  of  this  subject  in  a  subsequent  chapter 
makes  it  unnecessary  to  pursue  it  much  further  in  this  one. 

Intrinsic  Aberrations.— In  the  eyeball  itself  are  : 

(1)  Angle  alpha  of  Danders. — The  angle  between  the  antero- 
posterior  axis  of  the  eyeball  (which  Bonders  assumed  wrongly  to 
coincide  with  the  axis  of  the  cornea)  and  the  visual  line.     Varia- 
tions of  this  angle  cause  deceptive  appearances  of  squint. 

(2)  Angle  alpha  of  Landolt. — The  angle  between  the  visual 
line  and  the  major  axis  of  the  corneal  ellipsoid.     This  angle  con- 
tributes nothing  to  a  deceptive  appearance  of  squint. 

(3)  Angle  gamma. — The  angle  between  the  antero-posterior 
axis  of  the  eyeball  and  the  fixation  line.     This  angle  differs   but 
slightly  from  the  last,  since  the  fixation  line  so  nearly  coincides 
with  the  visual  line  and  therefore  has  a  bearing  on  apparent  squint. 
The  fixation  line  proceeds  from  the  point  of  fixation  to  the  center 
of  motion  of  the  eyeball.     The  visual  line  also  proceeds  from  the 
point  of  fixation  but  to  the  anterior  nodal  point. 

Linear  Strabismometry. — This  method  of  measuring  squint 
has  almost  died  out  of  use.  It  took  account  of  the  linear  dis- 
placement of  the  pupil  and  was,  at  one  time,  the  popular  method, 
owing  to  Graefe's  view  that  a  displacement  of  the  pupil,  measured 
by  so  many  lines  or  millimeters,  could  be  rectified  by  setting  back 
the  tendon  by  an  equal  number  of  lines  or  millimeters.  In  practice, 
however,  this  has  been  found  to  be  impossible,  and  Landolt  pointed 
out  that,  owing  to  the  different  lengths  of  different  eyes,  an  angular 
measurement  was  the  only  rational  one. 

Nevertheless,  the  linear  method  did  good  service  in  its  day,  and 
flat  pieces  of  ivory  with  a  concavity  to  fit  the  lower  lid  are  still  to 
be  met  with,  being  relics,  more  or  less  faithful  to  the  original,  of 
Lawrence's  strabismometer,  once  much  used  in  England.  The 
concavity  is  graduated  in  millimeters  from  a  central  zero  and  is 
intended  to  be  used  in  this  way  :  Place  the  patient  facing  the  window 
and,  with  the  good  eye  covered,  direct  his  attention  to  some  distant 
object.  Place  the  zero  of  the  scale  just  under  the  pupil  of  the  now 
straight  but  usually  squinting  eye,  and  then  uncover  the  better  eye  : 
at  once  the  squinting  eye  asserts  its  habit  and  the  figure  which  now 
lies  under  its  pupil  measures  the  squint.  The  relation  between 


Strabismus  139 

angular  and  linear  measurements  may  be  expressed  by  saying  that 
each  millimeter  along  the  sclerotic  means  about  4^2°  of  squint. 

Hirschberg'S  Method.— A  lighted  candle  is  held  one  foot  in  front 
of  the  patient's  face,  the  surgeon  placing  his  own  eye  near  to  the 
candle  and  looking  just  over  it  at  the  eyes  of  the  patient,  who  is 
made  to  look  at  the  candle.  The  position  of  the  corneal  reflection 
on  the  squinting  eye  indicates  roughly  the  amount  of  squint.  Since 
the  breadth  of  the  cornea  is  about  12  mm.,  a  squint  which  brings  the 
reflection  to  the  margin  of  the  cornea  is  one  whose  linear  measure- 
ment is  half  the  diameter  of  the  cornea,  namely,  6  mm.  Half  this 
displacement  means  a  squint  of  3  mm.,  and  so  on.  Hirschberg 
points  out  that  a  6-mm.  squint  in  which  the  reflection  occupies  the 
margin  of  the  cornea,  means  one  of  about  45°,  while  one  in  which 
the  reflection  occupies  the  margin  of  a  medium  pupil,  is  about  15°. 

Owing  to  the  angle  gamma  between  the  optic  axis  and  fixation 
line,  w^hich  the  Hirschberg  method  neglects,  a  reflection  situated 
over  the  outer  margin  of  an  average  pupil,  means  a  greater  and 
sometimes  a  much  greater  squint  than  a  reflection  situated  over  the 
inner  margin.  Nevertheless,  Hirschberg' s  method,  as  far  as  it 
goes,  is  a  very  useful  one  and  often  enables  an  excellent  guess  to  be 
made,  provided  the  precaution  be  taken  to  keep  the  surgeon's  eye, 
the  flame  and  the  squinting  eye  in  one  straight  line.  If  not  quite 
so  accurate  as  the  use  of  ophthalmoscopic  corneal  images,  which 
were  introduced  at  a  much  later  date,  it  is  nearly  so,  and  it  possesses 
the  advantage  that  ' '  lights  ' '  are  generally  found  more  readily  than 
ophthalmoscopes.  For  the  more  exact  measurement  of  squint, 
however,  one  of  the  following  methods  is  necessary  : 

Perimeter  Method. — This  mode  of  measurement  assumed  sway 
as  soon  as  the  linear  method  began  to  wane  and,  in  the  manner 
recommended  by  Javal,  has  been  greatly  used,  its  advantage  being 
its  accuracy,  and  its  two  disadvantages  lying  in  the  absorption  of 
time  by  the  preliminary  arrangements  and  in  the  difficulty  of 
measuring  slight  convergent  squints  by  it,  since  for  them  the 
surgeon's  head  interferes  with  the  fixation  line  of  the  sound  eye. 

The  patient  should  be  seated  so  as  to  bring  the  squinting  eye 
(S,  Fig.  46)  into  the  center  of  the  perimeter,  while  straight  in  front, 
at  a  distance  of  five  meters,  is  placed  a  candle  for  the  fixing  eye  F 
to  look  at.  It  is  only  some  perimeters  which  permit  this.* 


*The  useful  addition  to  the  perimeter,  introduced  by  Landolt  and  now  become  general, 
namely,  a  piece  of  soft  wood  to  be  gripped  by  the  teeth,  is  very  useful  for  strabisuiometry. 


140  Tests  and  Studies  of  the  Ocular  Muscles 

Another  flame,  or,  better  still,  a  small  electric  light,  is  then  moved 
along  the  arc  of  the  perimeter,  with  the  surgeon's  eye  ever  behind 
it,  till  its  reflection  appears  to  occupy  the  center  of  the  cornea  or 

rather  that  part  of  the  cornea 
which  our  knowledge  of  the 
angle  gamma  leads  us  to  select 
and  which  I  have  called  else- 
where the  "fixation-position" 
(Chapter  XI).  The  squint  is 
now  measured  by  that  figure  on 
the  perimetric  arc  which  lies 
against  the  flame. 

For  great  accuracy  the 
angle  gamma  can  be  measured 
separately  by  screening  the 
good  eye  and  making  the 
squinting  one  fix  the  ivory  disk 
of  the  perimeter  while  a  flame 
(with  the  surgeon's  eye  kept 
strictly  in  line  behind  it)  is 
moved  along  the  arc  till  its 

reflection  appears  to  occupy  the  exact  center  of  the  corneal  circum- 
ference. The  figure  reached  by  the  candle  enables  the  angle  gamma 
to  be  at  once  read  off  from  the  perimeter.  This  angle  should  be 
subtracted  from  the  record  of  a  convergent  squint,  and  added  to 
that  of  a  divergent,  premising,  of  course,  that  they  have  been 
measured  by  the  center  of  the  cornea. 

In  the  figure,  d  is  the  ivory  disk  of  the  perimeter,  and  were 
there  no  squint,  the  visual  axis  of  the  left  eye  would  pass  through 
this,  as  shown  by  the  dotted  line.  The  angle  d  S  /,  therefore,  is 
the  angle  of  the  squint  and  is  measured  by  the  arc  d  I. 

By  bringing  the  distant  flame  nearer  to  the  perimeter  the  squint 
can  be  measured  under  different  accommodative  conditions  ;  or, 
finally,  the  ivory  disk  may  itself  be  made  the  object  of  fixation. 

The  figure  makes  evident,  also,  how  in  a  squint  of  low  degree 
the  method  is  rendered  impracticable  by  both  the  flame  E  and  the 
surgeon's  head  behind  it,  interfering  with  the  vision  of  the  distant 
flame  by  the  fixing  eye  F. 

Charpentier's  Method.— The  difficulty  just  spoken  of  is  evaded 
in  the  plan  illustrated  in  Fig.  47,  where  advantage  is  taken  of  the 


Fig.  46 

Javal's  method. 


Fig.  47 

Charpentier's  method. 


Strabismus 


141 


law  that  angles  of  incidence  and  reflection  are  equal.  The  flame  is 
placed  over  the  fixation  spot  of  the  perimeter,  and  the  surgeon's 
eye  is  made  to  travel  along  the  arc  till  its  reflection  appears  to  lie  in 
the  center  of  the  cornea  of  the  squinting  eye.  The  squint  is  then 
measured,  its  angle  being  half  the  angle  of  the  arc. 

One  little  fallacy  in  this  test  seems  to  have  escaped  notice,  and  is  illus- 
trated in  Fig.  48.  Owing  to  "spherical  aberration,"  the  image  formed  by 
reflection  from  a  convex  mirror  alters  its  position  with  every  change  in  the 
angle  of  incidence  and  its  ever  equal  angle  of  reflection,  so  as  to  lie  on 
the  caustic  curve  shown  in  the  figure.  This  caustic  curve  is  one  whose 
cusp  F  lies  in  a  line  drawn  from  the  center  of  curvature  O  of  the  cornea 
parallel  to  the  incident  pencil  i.  By  producing  the  reflected  pencil  r  back- 
wards till  it  meets  the  caustic  curve  at  F'  the  position  of  the  image 
is  found. 

Moreover,  secondly,  the  sur- 
geon judges  by  its  projection 
against  the  plane  of  the  iris,  not 
against  the  center  of  the  cornea  ; 
so  the  while  the  image  lies  at  Ff 
on  the  caustic  curve,  it  is  pro- 
jected on  to  the  plane  of  the  iris, 
where  it  clearly  would  appear 
eccentric. 

In  the  absence  of  a 
candle,  a  circle  of  very  white 
paper,  mounted  in  the  peri- 
meter, gives  a  recognizable 
reflection  from  the  cornea, 
and  this,  indeed,  has  been 
utilized  in  De  Wecker  and 

Masselon's  "Arc  Keratoscopique,"  which  also  has  a  little  mirror 
in  which  the  distant  object  is  reflected  which  serves  for  the 
point  of  fixation.  It  seems  superfluous,  however,  to  have  a  special 
apparatus  for  strabismometry,  if  instruments  already  in  possession 
for  other  purposes  serve  as  well.  To  any  who  think  otherwise,  the 
"arc  keratoscopique "  will  be  found  very  handy. 

Priestley  Smith's  tape  method  will  be  found  described  in  the 
chapter  on  "  Ophthalmoscopic  Corneal  Images." 

The  Tangent  Strabismometer.— The  tangent  scale  (latest 
edition  best)  constructed  by  the  author  for  use  with  his  rod  test 
is  the  only  apparatus  needed,  and  since  it  hangs  on  the  wall,  it 
occupies  no  space  in  the  room  and  is  ever  ready.  It  is  in  principle 


Fig  48 

Section  of  Cornea,  to  show  that  light  reflected 
from  the  center  of  the  cornea  symmetrically 
does  not  produce  an  image  in  the  optic  axis, but 
rather  to  one  side  of  the  center  of  the  pupil. 


142 


Tests  and  Studies  of  the  Ocular  Muscles 


a  flattened-out  perimeter,  but  has  the  advantage  over  the  perimeter 
of  being  time-saving.  An  immense  number  of  squints  are  operated 
on  without  being  measured  in  degrees  simply  for  want  of  time,  and 
the  tangent  strabismometer  is  meant  to  meet  this  difficulty.  It  serves 


Fig.  49 

First  step  in  tangent  strabismometry  ;  adjusting  distance  of  patient  by  a  meter-string. 

for  both  the  objective  and  the  subjective  measurement  of  squint, 
and  whenever  diplopia  can  be  elicited,  both  kinds  of  tests  should  be 
made  ;  for  while,  on  the  one  hand,  subjective  measurement  is  far 
more  delicate  than  any  objective  measurement  could  possibly  be,  on 
the  other  hand  there  is  the  chance  of  meeting  with  one  of  those  occa- 
sional fallacies  in  the  projection  of  the  false  image,  which  need  check- 
ing by  the  rougher  though  more  dependable  objective  observations. 

The  large  figures  on  the  tangent  scale  are  not  intended  for 
ordinary  strabismometry,  but  rather  for  latent  deviations  or  slight 
squints  (under  10°),  since  they  represent  degrees  for  a  distance  of 
5  meters.  The  row  of  smaller  figures  is  added  for  strabismometry 
and,  being  intended  for  use  at  one  meter,  since  they  mark  degrees 
at  that  distance,  a  piece  of  string  one  meter  long  hangs  from  the 
candle  ready  to  adjust  the  distance  of  the  patient. 

Mode  Of  Use. — Placing  the  patient  facing  the  candle,  at  the 
meter  distance,  the  surgeon  introduces  his  own  head  between  the 
two,  but  a  little  lower  down,  about  a  foot  away  from  the  patient  and 


Strabismus  143 

so  that  the  root  of  his  own  nose  is  vertically  under  the  rays  of  light 
which  proceed  to  the  patient's  eyes.  At  once  the  tell-tale  corneal 
reflections  reveal  which  eye  is  the  squinting  one,  and  the  amount  of 
squint  being  guessed  at  by  the  degree  of  eccentricity  of  the  reflec- 
tion, on  Hirschberg's  principle,  the  patient  is  told  to  look  at  the 
figure  which  numerates  the  guess.* 

If  the  guess  of  the  amount  of  squint,  as  revealed  by  the 
corneal  reflections,  be  true,  the  squinting  eye  has  been  brought 
straight  for  the  candle  and  the  reflection  upon  it  occupies  its  proper 
position. 

If  the  guess  be  only  partially  correct,  successive  figures  are 
mentioned,  one  by  one,  for  the  patient  to  look  at,  till  the  surgeon 
is  satisfied  as  to  the  right  one.  For  rapid  work  this  suffices  and 
takes  scarcely  more  than  half  a  minute.  Since  the  height  of  the 
patient  is  immaterial,  no  time  is  lost  in  adjusting  it,  as  in  the  use  of 
the  perimeter. 

Greater  accuracy  still  may  be  secured  by  screening  the  work- 
ing eye  and  making  the  squinting  one  fix  the  flame  for  a  moment 
to  see  what  the  fixation-position  of  the  corneal  reflection  is  and 
whether  it  is  similar  to  that  of  the  working  eye. 

(i)  Concomitancy  can  be  measured  by  repeating  the  observa- 
tion with  the  patient's  face  turned  to  one  side  and  the  other  (Berry). 


Fig.   50 

Tangent  Strabismometry.     To  show  the  path  of  the  light  from  the  candle. 

(2)  The  secondary  deviation  can  be  measured  by  screening 
the  working  eye  and  making  the  squinting  eye  (and  the  face)  look 
at  a  figure  which  brings  the  working  eye  straight  for  the  candle,  as 
proved  by  momentary  unscreening  to  look  at  the  corneal  reflection  ; 
or  by  a  little  adeptness  the  working  eye  can  be  screened  from  the 
figure  looked  at  by  the  other  eye,  yet  not  from  the  flame. 


*According  to  Ilirschherg,  when  the  reflection  occupies  the  margin  of  a  moderate-sized 
pupil  then'  is  about  ln°  or  2<i°  of  squint  ;  when  the  margin  ot"  the  cornea,  about  45°.  For 
divergent  strabismus  the  figures  are  less.  The  angle  gamma  has  to  be  considered  always. 


144  Tests  and  Studies  of  the  Ocular  Muscles 

(3)  The  angle  gamma  can  be  measured  by  making  the  eye 
look  at   the  figure  which  brings  the  reflection    precisely  into  the 
center  of  the  cornea. 

(4)  The  degree  of  eccentric  fixation  (or  the  position  of  the  ' '  false 
macula"  of  some  authors)  is  measured  by  subtracting  the  angle 
gamma   of   the  sound   eye   from    the    apparent  angle    gamma    of 
the  squinting  one. 

(5)  Imperfect  abduction   of   the   squinting   eye    can    also   be 
measured  by  rotating  the  head. 


Tig.  51 

Second  step  ;  examining  the  refraction.    Also  estimating  the  vertical  element. 

(6)  The  accommodative  element  can  be  eliminated  by  holding 
the  correcting  lenses  before  the  fixing  eye.      Or  its  increase  can  be 
tested  by  holding  a  minus  lens  similarly. 

(7)  Any  vertical  elements  in  the  squint  can  be  measured  by 
temporary   adjustment   of    a   vertical   scale,  as    in    Fig.    51,    after 
measuring  the  horizontal  element ;  the  vertical  scale  being  placed 
under  the  figure  on  the  horizontal  scale  which  has  been  previously 
settled  on,  the  patient  being  made  now  to  look  at  that  figure  on  the 
vertical  scale  which  brings  the  reflection  to  the  "fixation-position."* 

*  Though  not  necessary  in  practice,  a  little  calculation  would  have  to  be  made  in  scientific 
inquiries,  to  correct  the  variable  angles  subtended  by  the  figure  in  the  vertical  scale  in  different 
situations. 


Strabismus  145 

The  rapidity  of  measurement  by  this  method  favors  the  good 
practice  of  measuring  squints  before  and  after  operation,  which  is 
generally  at  present  dispensed  with  to  save  time.  Another  advan- 
tage is  that  it  leaves  both  hands  free  to  analyze  the  squint.*  The 
average  effect  produced  by  a  tenotomy  differs,  of  course,  with 
different  operators  and  in  different  cases,  but  about  16°  may  be 
reckoned  a  good  effect  (Berry).  Expressed  in  linear  measure, 
this  would  be  from  3  to  4  mm. 

Worth's  Deviometer. — This  ingenious  modification  of  the  above 
method  adapts  it  admirably  to  tiny  children.  An  arm,  like  that  of 
a  signpost,  on  a  little  stand  can  be  swung  so  as  to  point  either  to 
the  right  or  to  the  left  and  answers  to  the  tangent  scale,  while  a 
brass  carrier  can  be  moved  along  it,  by  the  tapping  of  which  the 
child's  attention  is  attracted.  An  electric  light,  elongated  vertically, 
at  the  top  of  the  stand  is  governed  by  a  small  bell-push  to  enable 
it  to  be  flashed  on  instantaneously  and  off  again  before  the  child  has 
time  to  look  at  it.  The  instrument  is  placed  on  a  table  with  the 
nurse  seated  in  front  of  it  holding  the  child  on  her  knees.  Passing 
a  ring  on  to  her  finger,  to  which  is  attached  a  string  60  centimeters 
long,  from  the  center  of  the  instrument,  she  holds  the  child's  head 
steady.  The  surgeon,  placed  behind  the  upright  stand,  looks  over 
its  center  and  proceeds  as  described  in  the  previous  section,  first  of 
all  guessing  the  amount  of  squint  by  directing  attention  to  the 
electric  light  and  noticing  the  eccentricity  of  the  corneal  image  of 
the  squinting  eye  as  compared  with  its  position  in  the  good  one. 
The  brass  traveler  is  then  moved  along  to  that  position  which  repre- 
sents the  guess,  attention  being  attracted  to  it  by  a  tap  with  the 
finger  or  a  lighted  match,  and  the  guess  confirmed  or  rectified  as 
already  described  in  the  tangent-scale  method. 

Worth's  Modification  of  Sne lien's  Test. — For  children  who  have 
not  learned  their  letters,  Snellen's  well-known  test  with  colored 
glasses  for  ascertaining  the  presence  of  binocular  vision,  is  not  avail- 
able, and  Worth's  "four-dot  test"  is  an  excellent  substitute.  It 
consists  of  four  translucent  disks  set  in  a  black  background.  Each 
disk  is  three  inches  in  diameter  and  of  ground  glass,  the  upper  one 
being  red,  the  lower  one  white  and  the  two  intermediate  ones  green. 
The  patient,  armed  with  Snellen's  frame,  is  placed  five  or  six  yards 
away.  If  the  trial  frame  be  adjusted  with  a  red  glass  before  the 

*  Claud  Worth  has  added  to  the  little  device,  of  making  the  light  be  turned  on  and  ofl 
suddenly  when  using  this  test  for  babies. 


146  Tests  and  Studies  of  the  Ocular  Muscles 

right  eye  and  a  green  glass  before  the  left  eye,  and  the  patient  sees 
only  two  dots,  i.  e.,  the  white  and  the  red,  he  is  using  the  right  eye 
only.  If  he  sees  three  dots,  i.  e.,  the  white  and  two  green,  he  is 
using  the  left  eye  only.  If  he  sees  four  dots,  i.  e.,  all  the  four,  he 
has  simultaneous  perception  with  both  eyes,  and  fusion.  If  five 
dots,  however,  from  the  white  appearing  double,  he  has  simulta- 
neous perception  without  fusion,  or,  at  least,  without  fusion  when 
the  two  images  of  an  object  are  differently  colored.  This  is  the 
one  weak  point  of  the  test,  for,  needless  to  say,  when  the  white 
disk  is  resolved  into  two  they  are  no  longer  white,  but  are  tinted- 
one  green  and  the  other  red — by  the  glasses  of  the  trial  frame. 

Worth's  Treatment  of  Infantile  Squint. — By  far  the  most 
important  of  the  recent  advances  in  this  subject  have  been  made 
by  Worth,  who  has  shown  with  what  facility  squint  can  be  treated 
under  the  age  of  five  or  six  years  as  compared  with  their  treatment 
in  later  life,  over  which  so  much,  often  unavailing,  ingenuity  and 
pains  have  been  spent. 

(1)  Optical  correction  of  any  refractive  error,  of  course,  comes 
first.     After  thoroughly  atropizing  the  eyes  for  some  days,  he  cor- 
rects the  whole  astigmatism  and  all  but  .5  D.  of  the  hypermetropia, 
and  continues  the  mydriatic  until  the  glasses  are  made,  but  no  longer. 
So  far  this  is  as  others  do  ;  but  he  also  counts  that  "  no  infant  is  too 
young  to  bear  glasses  should  they  be  required,"  and  advocates  sides 
so  short  as  only  just  to  reach  above  the  ear  and  with  a  loop  at  the 
end  to  enable  them  to  be  tied  on.      In  the  proximity  of  the  loop  the 
sides  should  be  guarded  with  wool  twined  round  them  to  protect 
the  ear.     The  frequently-used  thin  plate  of  celluloid  or  tortoise  shell 
is  also  commended. 

(2)  Occlusion  of  the  Fixing  Eye. — If  the  vision  of  the  squint- 
ing eye  be  worse  than  76Tr,  he  advocates,  like  Javal,  continuous  occlu- 
sion, but,  since  he  deals  with  younger  children,  does  not  continue 
so  long  ;  at  the  most,  one  or  two  months.      He  fixes  on  a  gauze 
pad,  secured  by  bandage  or  plaster,  for  a  few  weeks,  followed  by 
cotton- wool  packed  behind  the  spectacle  lens.* 

(3)  Instillation    of  Atr opine    into   the  Fixing  Eye   Only. — 
Though  this  has  been  recommended  by  Priestley  Smith  and  others 
previously,  Worth  has  done  excellent  service  in  enforcing  the  value 
of  this  procedure  more  distinctly  than  heretofore  and  in  showing  up 

*The  author,  however,  prefers,  in  other  than  hospital  children,  gauze-surrounded  lenses 
tn  ground  glass  before  the  fixing  eye,  and  tied  on  with  ta)>e,  since  this  keeps  the  fixing  eve 
•e  cool  and  comfortable.    The  wire  gauze  should  have  a  velvet  edge 


Strabismus  147 

the  undoubtedly  worthless  plan,  practiced  by  a  few,  of  atropizing 
both  eyes.  He  prescribes  daily  morning  instillations  into  the  fixing 
eye,  adding  the  practical  procedure  of  giving  the  mother  a  card  on 
which  are  written  the  directions  and  the  date  of  the  next  visit. 
"The  best  results,"  he  adds,  "are  obtained  in  children  whq  are 
not  more  than  four  or  five  years  of  age.  After  six  years  of  age, 
usually  not  much  improvement  in  vision  can  be  obtained." 

(4)  Training  the  Fusion  Sense. — If  under  six  years  of  age, 
this  is  carried  out  by  Worth  by  the  use  of  his  "  amblyoscope,"  an 
ingenious  departure  from  Priestley  Smith's  fusion  tubes,  possessing 
the  novelty  of  a  mirror  in  each  tube,  which  facilitates  their  conver- 
gence, while  keeping  the  graphic  designs  so  far  apart  as  to  make  it 
easy  to  illuminate  the  one  seen  by  the  amblyopic  eye  more  brilliantly 
than  the  other,  by  means  of  two  lamps  or  electric  lights  placed  over 
against  them.      "The  favorable  time  for  fusion  training  is  between 
three  and  five  years." 

(5)  Operation. — For  cases  in  which  the  deviation  is  not  over- 
come by  other  means  ;  advancement  for  moderate  deviations,  com- 
bined with  tenotomy  in  the  higher  degrees. 

As  regards  tenotomy  for  convergent  squint,  the  plan  of  choosing 
the  squinting  eye  for  operation  is  doubtless  the  best  one,  since  it 
agrees  with  the  wishes  of  the  patient,  who  does  not  understand 
"conjugation."  Yet  I  have  an  impression  that  a  slightly-greater, 
effect  would  be  gained  by  tenotomizing  the  other. 

If  an  advancement  be  done,  it  is  better  performed  on  the 
external  rectus  of  the  squinting  eye,  especially  if  there  be  any 
deficient  abduction. 

In  divergent  strabismus  the  most  valuable  rule  to  remember  is 
that  if,  on  approaching  the  finger  towards  the  straight  eye,  no  con- 
verging effort  is  visible  in  the  diverging  eye,  tenotomy  is  an  entirely 
useless  procedure  ;  its  effect  will  be  nil.  Advancement  is  indicated. 

In  absolute  divergent  strabismus  my  experience  confirms  that  of 
Javal,  that  "  there  is  no  fear  of  producing  an  exaggerated  operative 
effect."  If,  after  a  surgical  interference,  a  little  convergence  is  left 
for  certain  directions  of  fixation,  this  effect  pretty  rapidly  disap- 
pears. "We  cannot,"  he  remarks,  "count  upon  a  durable  cure 
unless  optical  means  are  employed  immediately  after  the  operation. 
A  squint  is  not  definitely  suppressed  unless  the  subject  has  acquired 
the  habit  of  reading  binocularly.  When  we  have  to  do  with  an 
adult  whose  divergent  strabismus  has  become  permanent  for  an 


148 


Tests  and  Studies  of  the  Ocular  Muscles 


extremely  long  time,  even  the  most  successful  operation  must  be 
followed  by  the  stereoscopic  exercises  continued  several  hours  a 
day  for  months.  I  shall  quote  some  examples  of  success,"  he 
adds,  "  up  to  the  age  of  forty-five  years,  but  the  patients  say  with 
truth  that  the  remedy  is  worse  than  the  disease.  Even  with  females 
I  would  not  advise  undertaking  such  a  cure  after  the  age  of  twenty 
or  twenty-five  years.  With  young  girls  from  fifteen  to  twenty 
years,  on  the  contrary,  one  is  generally  seconded  by  a  courage 
and  a  patience  proof  against  everything,  and  success  is  absolutely 
assured. ' ' 

As  a  commentary  on  the  above  remarks  about  the  gradual 
decrease  of  operative  over-effect,  the  following  illustrative  account 
of  one  of  my  cases  will  be  of  interest : 

M.  L.,  school  girl;  myopia  corrected  by  — .5  D.  Left  eye  deviates 
outwards  when  tired.  By  objective  strabismometry  left  eye  diverges  20° 
(abbreviated  thus  :  —  20°  L.  Concomitant.  Very  low  angle  alpha. 

September  5th. — The  r.  and  1.  external  recti  were  tenotomized  under 
chloroform,  and  the  r.  internal  rectus  advanced.  Examination  by  the  glass 
rod  and  tangent  scale  gave  the  following  results  : 


DATE 

ON   LOOKING 
TO  RIGHT 

ON   LOOKING 
STRAIGHTFORWARD 

ON    LOOKING 
TO   LEFT 

September  I4th 

+    12° 

+     10° 

+  3° 

September  isth 

+    10° 

+     7° 

-    o° 

October  5th  .   . 

+     10° 

+     o° 

-  3° 

October  26th     . 

+     4° 

—       2° 

,0 

November  8th  . 

+     4° 

—       2° 

-  4° 

February  5th    . 

4     2° 

—       2° 

—    2° 

Subjective  Strabismometry. — The  subjective  test  is  made  by 
holding  a  disk  of  red  glass  rods  before  the  squinting  eye  and  read- 
ing off  that  figure  on  the  same  scale  which  appears  crossed  by  the 
streak  of  light.  A  piece  of  blue  or  green  glass  before  the  working 
eye  improves  the  effect  by  making  the  images  more  dissimilar  in  color 
and  more  equal  in  intensity.  (See  M.  L.'s  case,  above  cited.) 

Concomitancy  can  be  measured  by  turning  the  face  to  one  or 
other  side,  as  before,  and  comparing  the  readings. 

Direction  of  Fixation.— In  both  the  above  tests  the  patient's 
face  can,  if  desired,  be  turned  towards  the  figure  he  is  fixing,  so  as 


149 

to  gain  the  advantage  of  measurement  under  the  usual  conditions 
of  vision  with  the  fixing  eye  looking  straight  forward. 

Paralytic  Equilibrium. — So  far,  we  have  left  out  of  calculation 
the  modifying  effect  of  Tenon's  capsule  and  its  adnexa.  Let  us  now 
take  that  into  account  also.  Since  the  eyeball  is  so  nearly  spherical 
and  the  center  of  motion  so  nearly  at  its  geometrical  center,  we  may, 
with  little  error,  assume  that  they  are  quite  so  and  that  equal  forces 
have  equal  moments.  This  enables  us  to  say  that  when  a  single 
muscle  contracts,  the  tension  in  its  tendon  is  equal  and  opposite 
to  the  resultant  of  all  the  other  tensions,  of  which  there  are  two 
groups,  namely,  those  in  the  remaining  tendons  and  those  in  the 
orbital  fasciae. 

When  the  same  muscle,  however,  is  paralyzed,  the  eyeball  is 
under  the  influence  of  two  now  opposing  groups  of  tensions,  those 
of  the  fasciae  (which  tend  to  keep  it  in  the  primary  position*)  and 
those  of  the  tendons  of  the  still  unparalyzed  muscles  (which  tend 
to  rotate  it  away  from  the  primary  position).  The  resultant  of 
these  two  groups  is  equal  and  opposite  to  the  tension  which  existed 
during  health  in  the  paralyzed  muscle.  As  if  guided  by  this  resul- 
tant, therefore,  the  eyeball  rotates  in  the  opposite  direction  about 
the  same  axis. 

It  must  be  remembered  that,  in  paralyses,  though  the  belly  of 
the  affected  muscle  has  lost  its  contractility,  it  does  not  lose  its 
elasticity  at  once  and  in  some  pareses  does  not  wholly  lose  at  once 
even  all  its  physiological  tone,  so  that  the  new  position  into  which 
the  eye  settles  is  resisted  not  only  by  the  tension  in  Tenon's  capsule, 
but  also  by  the  remaining  elastic  tension  in  the  paralyzed  muscle. 

For  this  reason  paralysis  of  a  muscle  only  produces  a  very  slight 
effect  at  first,  while  the  healthy  eye  is  in  the  primary  position,  /".  e. , 
so  long  as  voluntary  innervations  are  quiescent. 

Secondary  Contracture,  or  Consecutive  Deviation. — But  as  time 
goes  on,  the  lamed  eye  deviates  more  and  more,  owing  to  the  loss 
of  vital  resistance  in  the  paralyzed  muscle,  to  which  wre  may  perhaps 
add  what  physicists  call  "fatigue  of  elasticity"  in  it  and  in  the 
resisting  portions  of  Tenon's  capsule.  Thus  arises  what  is  gener- 
ally called  "  contracture  of  the  antagonist."  Mr.  Berry  believes 
that  there  is  no  real  contracture.  I  am  inclined  to  believe  that  in 
the  course  of  years  a  slight  contracture  does  occur  in  the  opposing 
muscle  or  muscles,  but  as  a  consequence  rather  than  as  a  cause  of 

*  Probably  in  a  more  divergent  position  than  the  primary,  as  Hanson  Grut  has  shown. 


150  Tests  and  Studies  of  the  Ocular  Muscles 

the  increase  in  the  paralytic  deviation.  When  the  lame  muscle 
becomes  stretched  and  its  resistance  enfeebled  more  and  more,  the 
others  move  the  eyeball,  without  their,  however,  becoming  stronger 
than  they  were  before. 

My  impression  is  that  the  consecutive  deviation  (as  I  prefer  to 
call  it,  since  this  name  commits  to  no  theory)  will  be  found  great 
in  proportion  to — 

(1)  The  absoluteness  of  the  paralysis  ; 

(2)  The  long-standing  of  the  paralysis  ; 

(3)  In  proportion  as  the  paralytic  deviation  is  supplemented 
by  a  pre-existing  latent  deviation  ; 

(4)  In  proportion  to  the  degree  of  atrophy  of  the  paralyzed 
muscle  from  (a)  Want  of  innervation,  (<£)  Want  of  use  ; 

(5)  The  more   yielding   Tenon's   capsule   is,  and    the   more 
readily  it  experiences  fatigue  of  elasticity  ; 

(6)  The  more  the  habit  of  the  patient  is   to   turn    the   eyes 
away  from  the  side  of  the  paralyzed  muscle  ; 

(7)  The  more  the  patient  uses  the  paralyzed  eye  ; 

(8)  In  the  case  of  paralytic  convergent  strabismus,  the  greater 
the  hypermetropia  and  the  more  sensitive  the  converging  center  ; 
and  vice  versa  in  paralytic  divergent  strabismus. 


CHAPTER   VIII 


Ocular  Paralyses 

In  the  absence  of  any  visible  squint,  the  most  evident  symp- 
toms of  an  ocular  paralysis,  beginning  with  the  more  objective,  are  : 

(1)  Vicarious  inclination,  or  unusual  pose,  of  the  head; 

(2)  Imperfect  movement  of  an  eye  ; 

(3)  Magnified  secondary  deviation  ; 

(4)  Malprojection  ;  and,  under  certain  conditions, 

(5)  Giddiness,  and 

(6)  Uncertainty  of  gait ; 

(7)  Diplopia. 

(8)  In  addition  to  these  it  sometimes  happens  that  asthenopia, 
headache  and  a  strained  feeling  of  the  eyes  are  caused  by  the  con- 
tinual efforts  required  to  preserve  single  vision  in  the  presence  of  a 
slight  muscular  paresis,  though  care  must  be  taken  to  exclude  other 
more  likely  causes  of  these  symptoms. 

If  really  of  muscular  origin,  they  cease  when  the  attempt  to 
maintain  single  vision  is  given  up.  A  good  practical  test,  therefore, 
is  to  keep  the  suspected  eye  covered  for  a  sufficient  time  and  note 
whether  so  doing  causes  the  disappearance  of  the  symptoms. 

Let  us  now  discuss  each  symptom  in  detail. 

Symptom  No.  1:  Vicarious  Inclination  of  the  Head.— The 
object  of  posing  the  head  is  to  avoid  the  inconvenience  of  diplopia, 
so  that  these  two  symptoms  are  alternate. 

Whenever  the  eyes  look  in  a  direction  which  calls  for  activity 
in  the  paralyzed  muscle,  its  inefficiency  is  manifested  by  diplopia. 
•To  avoid  any  call  upon  the  muscle,  therefore,  the  patient  turns  his 
head  so  that  the  eyes  may  look  in  the  opposite  direction  to  that  of 
the  most  troublesome  diplopia.  It  was  called  "vicarious"  inclina- 
tion of  the  head  by  Graefe  because  the  neck  muscles  do  the  work 
instead  of  the  paralyzed  eye  muscle. 

Anyone  well  acquainted  with  the  subject  can  generally  guess 
the  associated  pair  of  muscles  of  which  one  is  paralyzed,  whenever 
a  patient  enters  the  room  with  a  marked  inclination  of  the  head. 
It  is  quite  easy  to  guess,  if  it  be  remembered  that  the  patient's  face 
looks  in  the  direction  of  the  paralytic  diplopia. 


I52  Tests  and  Studies  of  the  Ocular  Muscles 

There  are  six  directions  in  which  the  face  may  look  (if  we 
assume  that  a  single  muscle  only  is  affected),  and  each  of  these  six 
directions  is  in  relation  with  its  own  pair  of  muscles. 

Thus,  if  the  face  look  to  the  left,  one  of  the  two  kevoductors 
is  at  fault,  either  the  right  internal,  or  the  left  external,  rectus.  A 
face  directed  down  and  to  the  right  impeaches  the  dextral*  depres- 
sors ;  and  so  on. 

But,  after  all,  we  should  never  trust  implicitly  to  the  inclination 
of  the  head  without  proceeding  to  other  tests,  for  it  may  be  mislead- 
ing. A  fallacy  is  sometimes  introduced  by  the  fact  that  different 
components  of  the  diplopia  are  not  equally  troublesome  to  different 
patients.  Some  find  the  torsion  of  the  false  image  trouble  them 
disproportionately,  and  others  the  vertical  displacement ;  and,  since 
the  inclination  of  the  head  is  merely  adopted  by  the  patient  to 
avoid  embarrassment,  it  does  not  supply  mathematical  information. 
Some  patients,  indeed,  have  not  yet  discovered  the  best  inclination, 
and  need  to  have  it  pointed  out  to  them.  Differences  in  different 
patients  arise  chiefly  from  various  latent  conditions  of  equilibrium 
(heterophoria),  which  pre-existed. 

This  subject  may  be  closed  by  a  chart  of  the  positions  of  the 
face  as  follows  : 


IF  THE  FACE   LOOK 

THE  AFFECTED   MUSCLE 
IS   EITHER 

WHICH  ARE 

To  right  

R.  Ext.  R.  or  L.  Int.  R.    . 

Dextroductors 

To  left     --'. 

L.  Ext.  R.  or  R.  Int.  R.    . 

Laevoductors 

To  right  and  up    .    . 

R.  Sup.  R.  or  L.  Inf.  O.    . 

Dextral  t  elevators 

To  left  and  up  ... 
To  right  and  down  . 

L.  Sup.  R.  or  R.  Inf.  O.    . 
R.  Inf.    R.  or  L.  Sup.  O.  . 

Lseval  elevators 
Dextral  depressors 

To  left  and  down 

L.  Inf.    R.  or  R.  Sup.  O.  . 

Ljeval  depressors 

Symptom  No.  2 :  Imperfect  Movement  of  an  Eye.— Though  this 

may  be  due  to  some  obstruction  or  incease  of  resistance  as  by  a  tumor 
or  pterygium,  such  are,  in  practice,  too  evident  to  cause  any  mistake. 

*The  word  "dextral"  must  be  carefully  distinguished  from  " dextroducting."    Bv  a 
Jxtral  elevator"  we  do  not  mean  a  muscle  that  elevates  and  dextroducts,  but  one  that 
•Mwtei  most  when  the  eye  happens  to  be  dextroducted  by  another  muscle.    The  left  superior 
jue,  e.  g.,  is  a  Isevoductor  and  yet  a  dextral  depressor. 

It  must  not  be  forgotten  tha't  the  superductors  and  subductors  are  not  called  dfxtral  or 
because  of  turning  the  eyes  to  the  right  and  the  left,  but  because  their  vertical  effect  is 
hen  the  eyes  are  turned  to  the  right  or  to  the  left  by  other  muscles. 


Ocular  Paralyses  153 

Order  of  Examination. — It  is  good  order  to  test  first  the 
comparative  mobility  of  the  two  eyes,  with  the  conjugate  mobility 
of  both  together  ;  followed  by  the  examination  of  the  converging 
power,  and  ending  with  the  absolute  mobility  of  each. 

(1)  Comparative  Mobility. — Commencing,  then,  our  examina- 
tion by  testing  the  comparative  mobility  of  the  eyes,  we  make  the 
patient,  with  both  eyes,  follow  the  point  of  a  finger  as  it  is  moved 
upwards,  to  right  and  to  left,  and  intermediate  directions.      During 
these  manoeuvres  we  watch  both  eyes  closely  to  see  whether  they 
move  equally  in  every  direction,  or  whether  one  eye  tends  tc  linger 
or  "lag"   behind  the  other;   and   if   so,  in   which   direction   the 
lagging  is  most  apparent :  this  direction  will  invariably  be  found  to 
agree  with  the  direction  of  greatest  diplopia.* 

(2)  Conjugate  Mobility.— It  may  be,  however,  that  both  eyes 
are  equally  mobile  and  concomitant  and  yet  are  equally  defective 
in  their  movements  in  one  or  more  directions  ;  this  is  spoken  of  as  a 
defect  in  their  "conjugate  mobility."     For  instance,  on  attempting 
to  follow  the  finger  in  its  upward  path,  the  two  eyes  may  manifest  a 
perfectly  symmetrical  inability  to  rise  to  the  usual  elevation.    A  dis- 
tinctly less  common  condition  is  for  them  both  to  fail  in  their  move- 
ments to  the  right  or  to  the  left.     A  little  practice  is  required  to 
learn  the  normal  limits  of  movements,  in  order  to  decide  whether 
a  defect  of  this  kind  is  sufficiently  pronounced  to  be  considered 
pathological,  especially  as  a  good  deal  depends  on  the  amount  of 
effort  made  by  the  patient. 

Nystagmus  should  be  carefully  watched  for  at  the  limits  of  the 
motor  field ;  also  during  the  passage  of  the  finger  from  one  place 
to  another  any  jerky  or  irregular  movements  of  the  eyes  should 
receive  attention. 

(3)  Near  Point  of  Convergence. — The  object  of  the  fourth 
manoeuvre,  namely,  passing  the  finger  nail  towards  the  root  of  the 
nose,  is  to  estimate  the  power  of  "convergence."     Here,  again,  a 
little  practice  with  normal  eyes  is  all  that  is  required  to  learn  the 
average  converging   power,    though    the  result  will   be   found   to 
depend  a  good  deal  on  the  effort  made,  and  the  concentration  of 
the  attention.      Even  when  testing  a  patient  who    has  divergent 
squint,  the  estimation  of  converging  power  should  not  be  omitted, 
for  though  it  is  only  possible  for  one  eye  at  a  time  to  fix  the  finger, 

*It  is,  at  the  same  time,  well  to  notice  whether  the  lagging  eye  manifests  any  "  torsion  " 
in  its  ineffectual  effort  to  follow  the  sound  eye  up  or  down,  for,  if  it  does,  the  integrity  of  the 
oblique  muscle  which  causes  the  torsion  can  be  taken  as  proved. 


154 


Ttsls  and  Studies  of  the  Ocular  Muscles 


an  inward  movement  observed  in  the  other  eye  as  the  finger 
approaches  the  root  of  the  nose  affords  a  valuable  indication  that 
the  faculty  of  convergence  has  not  been  lost,  though  perhaps  for 
long  unused.  It  is  well  known  that  without  such  converging 

power,  tenotomy  of  the  external  rectus 
will  have  practically  no  effect  ;  but  \\ith 
a  fair  amount  of  it  remaining,  tenotomy 
may  be  undertaken  with  more  or  less 
prospect  of  success.  Should  greater 
exactness  be  required,  either  Landolt's 
well-known  dynamometer  can  be  used,  or 
simply  a  vertical  line  on  the  back  of  a 
visiting  card,  approached  to  the  patient's 
eyes  till  he  can  no  longer  by  any  effort 
keep  it  from  parting  into  two.  The 
shortest  distance  from  his  eyes,  measured 
by  a  dioptric  tape,  at  which  he  can  still 
see  single,  gives  the  number  of  meter 
angles  of  positive  convergence. 

(4)  Absolute  Mobility  of  Each  Eye.— 
We  may  next  find  the  greatest  possible 
excursion  of  which  each  eye  is  capable, 
while  covering  the  other,  by  invoking 
the  patient's  highest  voluntary  effort  to  follow  an  object  to  the 
extreme  limits  of  the  motor  field  in  all  directions.  The  value  of 
the  test  is  impaired  by  the  fact  that  voluntary  effort  is  such  a 
variable  quantity,  and  the  palpebral  aperture  by  which  we  judge 
the  extent  of  movement  is  liable  to  such  variations  of  size  and 
shape  in  different  individuals.  In  comparing  the  excursions  of 
the  two  eyes,  however,  these  disadvantages  are  reduced  to  their 
minimum. 

Under  normal  conditions  it  is  easy  to  make  the  outer  margin 
of  the  cornea  touch  the  outer  canthus  by  strong  abduction  of  the 
eye,  while  in  full  adduction  the  inner  margin  of  the  cornea  should 
be  slightly  buried  beneath  the  caruncle. 

Alfred  Graefe's  rule  is  that  the  inner  margin  of  a  moderately- 
dilated  pupil  should  be  brought  to  touch  an  imaginary  vertical 
line  ascending  from  the  lower  "  punctum  lachrymale. " 

While  inciting  these  extreme  movements,  watch  again  care- 
fully for  any  appearance  of  nystagmus,  and  if  it  should  seem 


Fig.  52 

Landolt'g  Dynamometer  for  esti- 
mating the  near  point  of 
convergence 


Ocular  Paralyses 


155 


desirable  to  repeat  the  test  with  more  approach  to  accuracy,  adopt 
LandolC 's  method  with  the  perimeter.  Place  the  patient's  head  so 
that  the  eye  under  examination  shall  lie  in  the  center  of  the  arc  of 
the  perimeter  ;  fix  the  head,  and  pass  a  small  piece  of  diamond 
type  along  the  arc  of  the  perimeter  till  the  patient  ceases  by  any 
rotation  of  the  eye  to  be  able  to  read  it.  If  the  eye  be  amblyopic, 
it  will  be  necessary  to  conduct  the  test  objectively,  which  can  be  done 
by  passing  a  small  lighted  candle  along  the  arc  of  the  perimeter  till 
its  reflection  occupies  the  "fixation  position"  on  the  cornea,  while 
the  patient  strives  his  utmost  to  look  to  that  side.  Schweigger's 
hand  perimeter  (Fig.  53)  would  be  the  most  convenient  for  this 
purpose  were  it  provided  with  a  strip  of  wood  for  the  patient  to  grip 
with  his  teeth. 

By  either  of  these  methods 
the  motor  field  can  be  plotted 
out  for  each  eye.  Its  limits  are 
a  little  greater  when  tested  ob- 
jectively than  when  tested  sub- 
jectively. An  excellent  sugges- 
tion by  Casey  Wood  is  to  fix  a 
strip  of  paper  with  a  row  of 
letters  on  it  to  the  perimeter, 
and  let  the  patient  read  along  the 
row  till  he  can  read  no  longer. 

Symptom  No.  3:  Dispropor- 
tionate Secondary  Deviation. — 
The  ''primary"  deviation  is 
that  which  is  found  in  the  par- 
alyzed eye  during  fixation  of  the 
good,  or,  as  it  has  been  called, 
the  "working"  eye.  It  occurs 
spontaneously  whenever  the  eyes 
look  in  the  direction  of  diplopia. 

The  "secondary"  deviation 
is  an  artificial  phenomenon  pro- 
duced by  screening  the  good  eye,  so  as  to  compel  the  paralyzed  one 
to  take  up  fixation  as  well  as  it  can.  The  effort  required  to  make  the 
paralyzed  muscle  contract  is  out  of  all  proportion  to  the  result,  and 
since  half  the  effort  must  go  to  the  other  eye,  its  deviation  becomes 
greatly  exaggerated  ;  this  is  then  called  the  secondary  deviation. 


Fig.  53 

Schweigger's  Hand  Perimeter 


156  Tests  and  Studies  of  the  Ocular  Muscles 

When  the  primary  deviation,  in  a  slight  paresis,  is  too  small  to 
be  discerned,  the  secondary  deviation  may  enable  a  diagnosis  to  be 
made,  but  to  obtain  its  full  effect  the  eyes  must  be  made  to  look  as 
far  as  they  can  in  the  direction  which  makes  the  greatest  demand 
on  the  suspected  muscle. 

Since  it  is  not  easy  to  see  the  behavior  of  the  good  eye  behind 
a  cover,  Javal  ingeniously  introduced  a  ground-glass  screen,  of  a 
circular  shape,  and  which  is  now  to  be  found  in  most  trial  cases  as  a 
companion  to  the  colored  disks. 

This  is  intended  to  be  held  as  close  as  possible  \.Q  the  good  eye, 
while  the  paralyzed  one  is  made  to  follow  the  surgeon's  finger  in 
the  direction  of  greatest  demand  on  the  muscle.  The  patient's 
eye  can  be  seen  through  the  ground  glass,  though  he  cannot 
himself  see  through  it,  and  the  secondary  deviation  can  be  quietly 
observed. 

Practically,  however,  the  obscured  disk  is  rarely  used,  because 
a  more  accurate  idea  of  the  deviation  is  obtained  by  suddenly 
withdrawing  the  hand,  or  some  quite  opaque  screen,  and  observing, 
first,  the  amount  of  deviation  ;  and,  secondly,  the  extent  of  the  visible 
corrective  movement  which  the  eye  makes  to  reclaim  its  fixation. 

When  the  affected  eye  has  the  best  vision  or  the  most  useful 
refraction,  the  patient  will  sometimes  still  use  it  as  the  working  eye, 
and  then  the  sound  eye  deviates.  These  cases  are  exceptions  to 
the  statement  that  the  secondary  deviation  is  artificially  created,  for 
the  patient  goes  about  exhibiting  it. 

Symptom  No.  4:  Malprojection. — This  never  occurs  except 
when  the  affected  eye  is  at  work,  either  alone  or  in  company  with 
the  other  eye.  If  alone,  the  malprojection  is  just  twice  as  great 
as  when  fixation  is  binocular.  The  principles  on  which  this  phe- 
nomenon are  based  have  been  gone  into  so  fully  in  earlier  pages 
that  little  need  be  added  here. 

For  Horizontal  Ductors.  — The  usual  plan  of  testing  is  to  make 
the  patient  cover  the  good  eye  with  a  hand,  and  then  suddenly  dart 
his  right-hand  forefinger  at  the  surgeon's  finger  held  upright,  at  an 
arm's  length  distance  from  the  patient,  in  such  a  position  as  to  make 
a  demand  upon  the  paralyzed  muscle. 

The  stab  must  not  be  a  slow  cautious  one,  neither  must  the 
patient  aim  with  his  finger  before  making  it.  He  will  miss  the  mark 
to  the  side  of  the  implicated  muscle  :  thus,  if  the  muscle  be  the  right 
external  rectus,  he  will  judge  the  surgeon's  finger  to  be  more  to  the 


Ocular  Paralyses  157 

right  than  it  really  is  and  will  miss  it  to  the  right,  really  stabbing  at 
a  phantom,  namely,  the  false  image,  the  reason  being  that  the  mind 
estimates  by  the  nervous  effort  expended  on  the  muscle,  as  if  the 
muscle  were  responding  to  it. 

I  have  nothing  to  add  to  the  usual  mode  of  performing  the  test, 
unless  that  after  the  patient  has  learned  to  correct  for  his  mistake, 
which  he  often  does  after  a  few  stabs,  it  is  interesting  to  uncover  the 
good  eye  and  cover  the  bad  to  see  if  he  now  at  first  misses  the  mark 
to  the  other  side.  A  few  attempts  with  each  eye  alternately  thus, 
makes  the  test  a  more  reliable  one. 

For  Vertical  Ductors. — When  the  affected  muscle  is  super-  or 
subductor,  the  projection  test  is  equally  simple  to  make.  The  sur- 
geon should  hold  his  finger  horizontally  above  the  horizontal  plane, 
if  the  muscle  be  a  superductor  ;  in  which  case  the  patient  will  aim 
too  high,  or  beiow  it  if  the  muscle  be  an  elevator,  when  the  patient 
will  aim  too  low. 

Symptoms  Nos.  B  and  6:  Giddiness  and  Uncertain  Gait. — The 
relation  of  these  symptoms  to  each  other  and  to  the  last  is  obvious. 
They  occur  only  when  demand  is  make  upon  the  paralyzed  muscle. 
Since,  in  the  case  of  the  ocular  muscles,  the  muscular  sense  is 
central  and  not  peripheral,  it  miscalculates  when  a  muscle  does  not 
truly  respond  to  its  stimulus.  It  is  when  depressor  muscles  are 
affected  that  the  inconvenience  reaches  its  maximum,  since  they  are 
needed  both  for  walk  and  for  work.  This  is  seen  frequently  in  the 
not  uncommon  paralyses  of  the  superior  oblique.  Covering  the 
affected  eye  stops  it  at  once,  and  sometimes  a  prism,  base  down 
before  the  weakened  eye,  and  another,  base  up,  before  the  good 
eye,  will  earn  the  hearty  thanks  of  the  patient.  Their  strength 
can  be  selected  after  an  examination  by  the  glass-rod  test,  and  the 
vertical  scale  (described  in  Chapter  XII). 

Symptom  NO.  <T:  Diplopia.— This  is  nearly  always  the  first 
symptom  of  which  a  patient  becomes  conscious.  At  the  earliest,  it 
is,  in  most  cases,  only  noticed  occasionally  ;  and  may  quite  disap- 
pear for  days  or  weeks,  to  return  again  in  a  more  marked  form. 
Later  it  becomes  sufficiently  established  to  appear  invariably  when- 
ever the  eyes  are  turned  in  some  particular  direction.  In  other 
cases  it  commences  suddenly  and  continues. 

Diplopia  is,  of  course,  absent  when  one  eye  is  nearly  blind, 
and  even  when  each  eye  has  good  visual  acuity  may  be  difficult  to 
realize  after  a  paralysis  has  lasted  some  years. 


158  Tests  and  Studies  of  the  Ocular  Muscles 

There  are,  however,  extraordinary  differences  among  patients 
in  the  persistence  of  the  diplopia,  some  learning  to  ignore  the  false 
image  in  a  few  months,  while  others  never  succeed  in  so  doing.  It 
is  most  evident  when  a  bright  light  is  looked  at  in  a  dark  room, 
and  I  find  that  a  piece  of  black  velvet  placed  behind  a  source  of 
light  so  greatly  enhances  its  apparent  brilliancy  as  to  aid  diplopia. 

EliCitable.— To  elicit  diplopia  when  the  patient  does  not  spon- 
taneously perceive  it  (a)  We  make  the  true  image  appear  different 
from  the  false  by  placing  a  colored  glass  before  the  good  eye. 
(b}  Another  plan  is  to  place  a  prism  before  the  affected  eye  so  as 
to  throw  the  image  on  an  unusual  part  of  the  retina,  (c)  When 
both  of  these  fail,  the  glass-rod  test  with  a  differently- colored 
disk  before  the  other  eye,  will  nearly  always  succeed  in  eliciting 
diplopia.* 

Monocular  Diplopia. — >In  practice  we  rely  chiefly  upon  the 
nature  of  the  diplopia  for  the  diagnosis  of  the  affected  muscle. 
The  first  step  is  to  make  sure  that  the  diplopia  is  not  monocular, 
by  covering  each  eye  in  turn  to  see  whether  one  image  disappears 
in  each  case.  The  image  which  disappears  belongs,  of  course,  to 
the  affected  eye. 

That  this  precaution  is  not  a  needless  one  may  be  shown  by 
the  fact  that  I  have  seen  a  case  of  monocular  diplopia  deceive  one 
of  the  best  of  surgeons.  The  case  was,  however,  peculiarly  decep- 
tive in  that  the  diplopia  was  noticed  by  the  patient  only  on  looking 
to  one  side.  By  the  employment  of  ophthalmoscopic  corneal 
images  afterwards  I  found  that  there  was  no  deviation  of  either 
eye,  in  any  direction  of  vision,  and  monocular  diplopia  was 
thereupon  searched  for  and  found. 

Common  but  Incorrect  Aphorism* — The  statement,  so  often 
made,  that  the  affected  muscle  is  the  one  which  physiologically 
turns  the  eye  in  the  direction  of  greatest  diplopia,  is  not  strictly 
correct.  Take  the  superior  rectus,  for  instance  :  its  greatest 
diplopia  when  paralyzed  is  up  and  out  ;  whereas,  its  physiolog- 
ical action  is  to  turn  the  eye  up  and  in. 

Corrected. — If  we  qualify  the  statement  by  saying  that  "the 
lame  muscle  is  one  which  in  health  turns  the  eye  in  the  cardinal 
direction  of  the  diplopia,"  it  becomes  at  once  unfailingly  true. 
The  cardinal  directions  are  up,  down,  right  and  left.  Diplopia, 

*I  generally  finft  it  best  to  place  the  glass  rod  before  the  good  eye,  with  or  without  a 
green  glas      efore  the  other,  the  source  of  light  being  brilliant,  and  backed  by  a  velvet  screen. 


Ocular  Paralyses  159 

greatest  in  the  upper  half  of  the  field,  is  undoubtedly  due  to  one  or 
more  of  the  elevators  ;  in  the  lower  half  to  one  of  the  depressors  : 
in  the  right  half  to  one  of  the  dextroductors  ;  and  in  the  left  half 
to  one  of  the  laevoductors.  *  There  can  be  no  mistake  here,  if 
mechanical  obstructions  are  excluded  ;  but  this  aphorism  only  helps 
us  to  find  the  group  to  which  the  affected  muscle'  belongs. 

Second  Aphorism. — Since  every  paralytic  deviation  makes  the 
false  image  travel  faithfully  in  the  opposite  direction  to  the  eye  by 
an  equal  angle,  and  since  also  the  physiological  displacement  of 
the  eye  by  the  muscle  before  the  paralysis  was  in  precisely  the 
opposite  direction  to  its  paralytic  deviation,  it  follows  that  the 
false  image  is  displaced  exactly  -as  the  healthy  muscle  originally 
displaced  the  eye. 

To  speak  figuratively,  when  the  muscle  fails  to  move  the  eye,  it 
moves  the  false  image  instead  in  the  same  direction  that  it  would 
have  moved  the  eye.  As  it  moves  the  image  in  disease,  it  moved 
the  eye  in  health. 

This  makes  it  very  easy  to  detect  the  muscle.  Is,  for  example, 
the  false  image  (relatively  to  the  true)  elevated,  adducted  and 
intorted  ?  Then  the  muscle  must  be  an  elevator,  adductor  and 
intortor.  Only  one  muscle  in  each  eye  is  this,  namely,  the  superior 
rectus  ;  so  the  case  is  solved. 

Complications. — If  there  were  no  complications,  this  "second 
aphorism"  would  suffice  for  all  our  need.  But  only  a  part  of  the 
displacement  of  the  false  image  may  be  due  to  the  paralysis,  the 
remainder  being  the  result  of  latent  squint  (heterophoria)  which 
may  have  pre-existed  for  years,  though  now  set  free  by  the 
paralysis.  This  introduces  a  fallacious  element  and  requires  that 
we  should  so  make  our  tests  as  to  avoid  it. 

Again,  more  than  one  muscle  may  be  affected,  and  we  might, 
if  unwary,  be  caught  in  a  trap. 

It  is  better,  therefore,  to  reserve  the  "second  aphorism"  to 
the  end  of  our  investigation  and  use  it  only  for  confirmation.  Even 
then,  to  get  the  full  benefit  of  it,  account  must  be  taken  of  the 
direction  in  which  the  sound  eye  is  looking,  for  muscles  have  dif- 
ferent effects  in  different  positions  of  the  eyeball,  and  the  position 
in  which  the  muscle  is  most  valuable  is  that  in  which  its  loss  is  most 
felt,  and  the  paralytic  diplopia,  therefore,  is  greatest.  The  superior 
rectus,  for  instance,  is  a  more  efficient  elevator  when  the  eye  is 

*The  convenience  of  these  terms  will  at  once  be  perceived 


160  Tests  and  Studies  of  the  Ocular  Muscles 

abducted  to  start  with  ;  therefore,  in  abduction,  its  vertical  diplopia 
from  paralysis  is  greatest.  In  adduction  it  is  a  more  efficient 
intonor  ;  therefore,  in  this  position  of  the  eye,  its  torsional  diplopia 
from  paralysis  is  most  marked.  And  so  on. 

Clinical  Procedure. — For  clinical  work  we  must  employ  the 
method  which,  while  thoroughly  simple,  is  freest  from  pitfalls. 

Instead,  therefore,  of  merely  considering  the  one  displacement 
of  the  false  image,  we  should  investigate  separately  its  vertical, 
horizontal  and  torsional  components,  giving  to  each  its  relative 
value,  since  they  are  not  equally  trustworthy  for  diagnosis.  We 
have  to  weigh  the  evidence,  and  not  merely  count  it. 

Narrowing  Circles. — Instead  of  rushing  straight  for  our 
muscle,  we  reach  it  by  stages,  just  as  a  botanist  with  a  flower 
enquires  successively  into  its  natural  order,  its  genus  and  its 
species. 

(a)  Cardinal  Groups. — We  begin  by  finding  to  which  of  the 
four  cardinal  groups  the  muscle  belongs,  whether  that  of  the  eleva- 
tors, the  depressors,  the  dextroductors,  or  the  Isevoductors,  in  which 
group  a  paralysis  makes  the  diplopia  increase  respectively  upwards, 
downwards,  to  right  or  to  left.  If  two  or  more  groups  seem  affected, 
begin  with  the  worst,  not  forgetting  that  vertical  diplopia  is  rela- 
tively more  important  than  horizontal  diplopia,  since  the  latter,  if 
it  extended  all  across  the  field,  may  be  due  to  some  anomaly  of  the 
converging  center. 

The  most  convenient  test  object  is  the  ever-ready  white  handle 
of  an  ophthalmoscope,  and  it  is  quite  enough  in  simple  cases.  If, 
however,  the  false  image  be  faint,  or  the  patient  unobservant,  a 
colored  glass  before  the  sound  eye  may  be  necessary,  used  in  con- 
junction either  with  a  lighted  candle,  or  a  strip  of  white  paper 
mounted  on  black  velvet,*  to  obtain  a  contrast  effect. 

Place  the  patient  with  his  back  to  the  window,  and  charging 
him  to  hold  his  head  erect  and  follow  the  test  object  with  his  eyes, 
move  it  upwards,  downwards,  to  right  and  to  left,  over  the  surface 
of  an  imaginary  hemisphere,  of  which  his  head  is  the  center  and 
with  a  radius  of  about  a  meter. 

While  testing  the  horizontal  motions  of  the  eyes,  hold  the  handle 
of  the  ophthalmoscope  vertically,  but  in  testing  above  and  below, 
hold  it  horizontally,  since  in  these  positions  the  vertical  component 

*Tho  Weal  test  object  would  be  a  luminous  glass  rod  about  six  inches  long  and  mountea 
urainst  black  velvet. 


Ocular  Paralyses  161 

of  the  diplopia  is  the  most  important  and  it  is  more  readily  estimated 
by  a  horizontal  than  by  a  vertical  test  object.  If  the  diplopia  is 
found  only  on  looking  upwards,  there  is  some  defect  among  the  group 
of  sursumductors  ;  if  on  looking  downwards,  among  the  group  of 
deorsumductors  ;  if  to  the  right,  among  the  group  of  dextroductors  ; 
and  if  to  the  left,  among  the  group  of  laevoductors. 

(£)  Affected  Eye. — Having  found  the  group,  the  next  thing 
is  to  find  the  eye,  which  is  easily  done,  while  the  test  object  is  still 
held  in  the  area  of  maximum  diplopia,  by  rapidly  screening  one  eye 
two  or  three  times  in  succession  with  the  hand,  in  order  to  find 
which  image  belongs  to  the  screened  eye.  It  is,  of  course,  the 
image  which  disappears  and  reappears. 

The  image  which  lies  farthest  in  the  direction  of  increasing 
diplopia  belongs  to  the  paralyzed  eye. 

If  the  affected  muscle  be  an  internal  or  external  rectus,  our 
work  is  done  when  we  have  found  the  group  and  the  eye,  for  each 
eye  has  only  one  dextroductor  and  one  laevoductor. 

(Y)  Delinqueut  Vertical  Ductor.—  But  if  the  fault  be  among 
the  group  of  elevators  or  depressors,  one  more  step  is  needful, 
since  each  eye  has  a  pair  of  each,  of  which  one  is  ever  a  rectus  and 
the  other  an  oblique. 

The  next  thing  to  do  is,  while  holding  the  test  object  (itself 
horizontal)  in  the  diplopic  half  of  the  field  to  pass  it  first  to  the 
right  hand  and  then  to  the  left,  to  note  in  which  position  the  ver- 
tical component  of  the  diplopia  appears  greatest  to  the  patient. 
If,  on  looking  to  the  same  side  as  the  paralyzed  eye,  the  difference 
in  height  is  greater  than  on  looking  to  the  other  side,  the  affected 
muscle  is  a  rectus.  If  the  difference  in  height  is  greatest  on  look- 
ing to  the  side  of  the  sound  eye,  the  affected  muscle  is  an  oblique. 

If  we  do  not  know  which  eye  is  wrong,  we  may  still  decide  in 
the  same  way,  whether  the  affected  muscle  is  dextral  or  laeval  in  its 
action.* 

Dextral  and  Laeval. — It  is  very  easy  to  recall  which  muscles 
are  dextral  and  their  vertical  effect,  and  which  laeval,  since  the 
dextral  are  those  whose  tendons  point  to  the  right,  and  the  laeval 

*T!ie  reader  who  is  accustomed  to  speak  only  of  lateral  and  medial  elevators,  and  adduc- 
tion and  abduction,  may  possibly  challenge  the  change  to  dextral  and  heval.  dextroduction 
mid  leevoduction.  The  reason  tor  it  is  that  since  the  former  terms  refer  to  the  median  plane, 
students  and  beginners  make  fmjxi'nl  mistakes  when  the  tast  object  lies  to  the  opposite  side  of 
the  median  plane  from  the  affected  eye,  calling  an  adducted  image  an  ahducted,  and  soon. 
This  constant  liability  to  error  is  entirely  removed  by  the  change  of  terms  I  have  employed. 
It  would  not  have  been  made  otherwise.  But  this  is  not  all  :  the  change  of  terms  enables  us  to 
adhere  more  closely  to  nature,  for  dcxtrodncting  and  Iwvoducting  innervations  exist,  but  we 
have  no  certain  knowledge  of  an  abducting  iunervation. 


162 


Tests  and  Studies  of  the  Ocular  Muscles 


those  which  point  to  the  left.  Fig.  54  makes  this  very  clear  and 
is  easily  borne  in  mind  after  a  few  moments'  contemplation  of  it, 
the  tendons  of  the  obliques  being  treated  as  if  they  pointed  forwards 

and  inwards,  instead  of  back- 
wards and  outwards. 

A  moment's  considera- 
tion will  show  how  it  must  be 
that  a  muscle  alters  the  height 
of  the  cornea  most  when  the 
visual  line  comes  to  lie  in  its 
muscular  plane. 

Since  a  dextral  muscle  is 
one  whose  muscular  plane 
points  to  the  right  and  a  laeval 
muscle  one  whose  muscular 
plane  points  to  the  left,  it  will 

have  their   directions 


Fig.  54 

To  show  how  those  Muscles  whose  vertical  effect 
is  dextral  have  Iheir    directions    pointing 


be  seen  that  the  recti  rightly 
describe   themselves,    for    the 
right  recti  are  dextral  in  their  action,  and  the  left  recti,  Iceval. 

The  obliques  are  contrary.     The  dextrals,   therefore,  are  the 
right  recti  and  the  left  ob- 
liques ;  the  laevafs,  the  left 
recti  and  the  right  obliques. 

There  is  no  need,  how- 
ever, to  commit  anything 
to  memory,  since  the  ana- 
tomical disposition  of  the 
muscles  can  always  be 
called  to  mind  sufficiently 
to  recollect  whether  its 
line  of  force  points  to  the 
right  or  to  the  left.  The 
attitude  oi  Fig.  55  may 
come  to  the  help  of  any 
one  unable  to  conjure  up 
the  muscles. 

Torsional  Purchase  and  Vertical  Purchase  Reciprocal.— 
While  the  vertical  purchase  of  a  muscle  is  greater  in  proportion 
as  its  muscular  plane  is  approached  by  the  visual  line,  its  torsional 
effect,  on  the  contrary,  increases  as  its  muscular  plane  is  departed 


Fig.  55 

Mnemonic  attitude  for  the  Muscular  Planes  (borrowed 
in  part  from  Landolt). 


Ocular  Paralyses  163 

from  by  the  visual  line.  Thus,  the  figure  shows  that  when  the  eyes 
look  to  the  right  the  dextrals  have  the  greatest  elevating  or  depress- 
ing effect,  and  the  laevals  have  the  greatest  torsional  effect;  and 
vice  versa  on  looking  to  the  left. 

The  corollary  is  that  the  greatest  torsion  of  the  false  image 
is  always  to  be  found  on  the  opposite  side  of  the  median  plane  from 
its  greatest  vertical  displacement,  z.  c. ,  if  the  greatest  vertical  sepa- 
ration is  up  and  to  the  right,  the  greatest  torsional  displacement 
will  be  up  and  to  the  left ;  or  if  one  is  down  and  to  the  right,  the 
other  will  be  down  and  to  the  left. 

As  soon  as  we  have  settled  whether  the  muscle  is  dextral  or 
laeval,  our  task  is  done,  and  the  diagnosis  made. 

Recapitulation. — Tc  summarize,  we  find*  : 
f  Elevators  ? 

s   ITT,  •  ,  Depressors? 

(1)  Which  group  <  —  . 

Dextroductors  ? 

^  Laevoductors  ? 

(2)  Which   eye?     The  eye   which   sees   the  most   advanced 
image  in  the  direction  of  diplopia. 

(3)  Rectus   or    oblique?     Rectus — if    the  maximum  vertical 
diplopia  be  on  the  side  of  the  paralyzed  eye.      Oblique — if  on  the 
side  of  the  sound  eye. 

Confirmation  of  the  Diagnosis.— It  is  well,  if  time  allow,  to 
study  the  three  components  of  the  diplopia  in  different  parts  of 
the  field. 

While  we  do  not  trust  much  to  the  torsion,  or  to  the  minor 
degrees  of  horizontal  diplopia,  in  discovering  the  muscles,  they  both, 
but  especially  the  former,  afford  valuable  confirmation  ;  and  if  the 
torsion  conflicts  with  our  discovery,  the  initial  investigation  should 
be  repeated. 

The  best  plan  for  confirmation  is  to  draw  up  a  motor  chart  in 
the  usual  way,  dividing  the  field  into  nine  areas,  as  showr  in 
Fig.  57,  and  carefully  representing  the  false  image  over  as  many  as 
it  appears  in. 

The  non-diplopic  areas  constitute  the  "field  of  single  vision," 
and  this  can,  if  desired,  be  also  filled  in  by  the  aid  of  the  glass  rod, 
though  hitherto  the  field  of  single  vision  has  been  generally  left 
unanalyzed. 

*  Personally,  I  prefer  to  find  the  eye  last,  but  have  adhered  to  the  usual  order  in  the  text  u 
being  more  easily  explained. 


and  Studies  of  the  Ocular  Muscles 

Measured  Charts.— If  more  accuracy  be  required,  the  three 
components  of  the  diplopia  (vertical,  horizontal  and  torsional) 
can  In-  iinusiirfd  in  degrees  tor  each  area  by  the  glass  rod  and  the 
tangent  scales. 

Construct,  then  Scrutinize.— It  is  well  to  fill  in  the  entire  chart 
before  reasoning  on  it,  so  as  to  be  unprejudiced  in  the  observations. 
Then,  see  if  the  false  image  corresponds  in  each  area  to  the  physio- 
logical action  of  the  suspected  paralyzed  muscle  during  vision  directed 
towards  that  area  For  this  we  recur  to  the  italicized  rule  previously 
enunciated.  Another  way  of  putting  it,  more  handy  than  elegant, 
is._"  What  the  muscle  does ;  the  false  image  is." 

Example. — For  example,  suppose  the  left  inferior  oblique  to  be 
implicated.  We  know  that  it  becomes  a  purer  and  stronger  sursum- 
ductor  on  looking  to  the  right  (Fig.  54).  The  false  image,  therefore, 
in  the  right  upper  part  of  the  field  will  be  more  purely  and  greatly 
sursumducted  above  the  true,  than  anywheie  else  (Fig.  68). 

We  know,  too,  that  it  still  has  some  laevo-torsional  purchase, 
even  on  looking  to  the  right,  and  that  it  still  slightly  laevoducts  the 
left  eye  ;  therefore,  the  false  image  will  be  laevotorted  and  slightly 
laevoducted.  accordingly,  in  that  same  area. 

We  know,  further,  that  on  looking  to  the  left  the  elevating 
power  ot  the  left  obliques  almost  ceases,  and  their  torsional  and 
abducting  effects  reach  their  maximum  ;  therefore,  in  the  left  upper 
area  of  the  field  the  false  image  will  be  but  slightly  higher  than  the 
true,  though  greatly  laevotorted  and  moderately  laevoducted.  In 
the  lower  part  of  the  motor  field  the  muscle  has  but  trifling  sway, 
and,  therefore,  here  diplopia  disappears,  the  false  image  running 
into  the  true. 

Names  Of  the  Areas.— In  drawing  up  the  chart,  let  everything 
be  denominated  by  the  patient's  right  and  left,  and  not  by  the 
surgeon's.  This  is  an  invariable  rule  for  everything  in  ophthal- 
mology. Fig.  74,  which  will  appear  in  its  numerical  order,  shows 
the  names  which  I  think  it  best  to  permanently  give  to  the  areas, 
in  view  of  a  namesake  principle  to  be  described  later  on.  They 
are  names  easily  recalled,  because  simply  descriptive  of  their  posi- 
tion from  the  patient's  point  of  view. 

Inscription.— The  mode  of  inscribing  the  false  image  or  the 
diplopia  is  a  matter  of  taste,  and  I  have  not  yet  settled  on  a  final 
choice.  My  favorite  way  at  present  is  to  represent  the  true  image 
by  a  dot  in  the  center  of  each  area,  leaving  the  imagination  to 


Ocular  Paralyses  165 

construct  a  vertical  line  through  the  dot.  Then  a  thin  vertical  line 
from  the  dot  represents  the  vertical  element,  a  horizontal  line  from 
the  end  of  that  the  horizontal  element,  and  a  larger  dot  at  the 
end  of  that  line  represents  the  false  image. 

The  advantage  of  this  plan  is  that  we  are  not  bound  to  inscribe 
the  torsion  if  the  patient's  account  of  it  is  unsatisfactory,  while  if  we 
do  wish  to  inscribe  it,  a  thin  line  through 
the  second  dot  shows  it    at    once   (as  in 
Fig.  56).      Moreover,  if  we  wish  to  make  a 
quantitative  record,  we  can  use  dotted  lines 
and  make  each  dot  represent  a  degree,  to  re- 
present the  horizontal  and  vertical  element  ; 
or  an  inch,  or  any  unit  we  like  to  choose. 
The  torsion  can  also  be  marked  in  degrees. 

Never  forget   to  record  on  the  chart  Fig  66 

to  which  eye  the  false  image  belongs. 

Even  Incorrect  Statements  are  Valuable,  if  True  Comparatively. 
— It  need  hardly  be  said  that  comparative  statements  about  the 
diplopia  in  the  different  areas  are  more  common  with  patients  than 
absolutely  true  measurements,  yet  though  the  patient's  idea  of  an 
inch  may  be  far  out,  it  does  not  matter  if  he  is  consistent  otherwise, 
and  maintains  his  peculiar  inch  throughout.  To  enable  him  to  do 
so,  care  should  be  taken  to  hold  the  test  object  at  the  same  distance 
from  the  eyes  throughout  the  test.  With  the  ophthalmoscope 
handle,  three  or  four  feet  is  a  convenient  distance  ;  with  a  candle, 
six  feet. 

If  more  than  one  muscle  be  affected,  the  diplopia  may  increase 
in  more  than  one  direction,  and  each  direction  may  then  be  studied 
independently.  Thus,  if  a  depressor  and  an  elevator  be  both 
paralyzed,  diplopia  will  increase  both  upwards  and  downwards,  and 
become  almost  nil  on  looking  straight  forward. 

In  dealing  with  multiple  paralyses,  a  careful  inscription  should 
be  made  in  every  area  of  the  chart,  without  bias  or  prejudice,  and 
then  the  affected  muscles  should  be  puzzled  out  from  it. 

To  Read  a  Simple  Cnart. — At  the  risk  of  being  tedious,  I  will 
give  one  example  of  a  single  paralysis.  An  inspection  of  Fig.  61  : 

(#)  Shows  diplopia  upwards ;  therefore,  involving  one  ot  the 
group  of  sursumductors. 

(6)  The  highest  image  (say)  belongs  to  the  right  eye  ;  there- 
fore, the  muscle  is  one  of  the  elevators  of  the  right  eye. 


1 66  Tests  and  Studies  of  the  Ocular  Muscles 

(r)  Its  maximum  vertical  diplopia  is  up  and  to  the  right ; 
therefore,  it  is  a  dextral  superductor.  But  there  is  only  one  such 
muscle  of  the  right  eye— the  superior  rectus.  Found,  therefore. 

Does  the  torsion  agree  ?  Yes  ;  for  though  there  is  none  ia 
the  right  superior  area,  there  is  marked  laevotorsion  in  the  left 
superior.  Had  it  been  a  case  of  the  right  inferior  oblique,  the 
greatest  elevation  would  have  been  to  the  left  side,  and  the  greatest 
torsion  to  the  right  side  ;  moreover,  the  torsion  would  have  been 
dextrotorsion.  The  diagnosis  is  confirmed,  therefore. 

TO  Read  a  Multiple  Chart.— (a)  Begin  by  noticing  in  how 
many  and  which  cardinal  directions  the  diplopia  seems  to  increase, 
and  if  it  does  so  in  more  than  one  direction,  begin  with  that  of 
greatest  diplopia.  Observe  which  group  this  greatest  diplopia 
points  to.  (£)  If  the  observed  diplopia  be  horizontal,  the  muscle 
is  found,  for  the  image  most  removed  from  the  center  of  the  chart 
belongs  to  the  affected  eye.  (*:)  If  the  diplopia  be  vertical,  the 
muscle  affected  is  a  rectus,  if  the  area  of  greatest  vertical  diplopia  is 
on  the  same  side  as  the  eye  that  sees  the  false  image  :  it  is  an 
oblique  if  on  the  opposite  side. 

Next  find  the  direction  of  second  greatest  (independent)  diplopia 
and  study  that  in  the  same  way.  Then  the  third,  and  so  on. 

Independent  Diplopiae. — Diplopise  in  opposite  halves  of  the 
motor  field  in  which  the  false  image  occupies  opposite  sides  of  the 
true  image  are  independent. 

If  the  false  image  remain  on  the  same  side  as  the  true  all 
across  the  field,  then  it  is  not  a  case  of  two  independent  diplopiae, 
but  there  is  a  concomitant  element,  due  either  to  an  anomaly 
of  the  converging  innervation  or  to  what  is  generally  called  the 
"secondary  contracture."* 

If  the  separation  of  the  images  is  constant  in  amount,  the 
diplopia  is  entirely  concomitant  ;  but  if  it  differs  in  degree  in 
different  areas  while  ever  the  same  in  kind,  there  is  a  paralytic 
element  as  well  as  a  concomitant  one. 

Concomitant  elements  are  distinguished  by  pervading  the  whole 
field  and,  therefore,  an  investigation  of  every  area  in  the  field  of  single 
vision,  as  by  the  rod  test,  leads  to  a  fair  estimate  of  their  amount. 

In  all  multiple  paralyses  the  diplopia  produced  by  one  muscle 
may  a'ter  that  due  to  another,  so  that  any  untypical  features  of 

•Theconconiitancy  of  'secondary  eontracture,"  or ''consecutive  deviation,"  as  it  is  better 
di  SOD      **  Vety  lml>er'ectl  the  <ievlat'0'»  becoming  less  and  less  towards  the  limits  of  minimum 


Ocular  Paralyses 


167 


diplopia  should  be  examined  to  see  whence  the  disturbance  from 
the  typical  proceeds,  just  as  an  astronomer  discovers  planets 
unknown  to  him  by  observing  the  disturbances  of  those  he  does 
know.  Those  who  have  not  experienced  it  are  little  aware  of  the 
difficulties  that  sometimes  attend  the  analysis  of  multiple  paralysis, 
with  much  heterophoria. 

Guides. — With  regard  to  the  direction  of  greatest  diplopia,  it 
is  useful  to  bear  in  mind  the  following  three  guides  to  it,  which, 
however,  are  only  roughly  true  : 

(1)  The  face  looks  at  it. 

(2)  The  affected  eye  lags  from  it. 

(3)  The  false  image  travels  towards  it. 

The  following  chart  classifies  the  twelve  ocular  muscles,  not 
anatomically  but  physiologically,  and  as  we  have  to  study  them 

clinically  : 

r 
2  Dextral 


4  Sursumductors 


4  Deorsumductors 


2  Laeval 


2  Dextral 
2  Laeval 


2  Dextroductors 


2  Lczvoductors 


R.  Sup.  Rectus. 

L.  Inf.     Oblique. 
(  L.  Sup.  Rectus. 
}  R.  Inf.     Oblique. 

R.  Inf.     Rectus. 

L.  Sup.  Oblique. 

L.  Inf.    Rectus. 

R.  Sup.  Oblique. 

R.  Ext.  Rectus. 

L.  Int.    Rectus. 

L.  Ext.  Rectus. 

R.  Int.    Rectus. 


It  will  be  seen  that  the  six  pairs  in  the  right-hand  column  are 
Graefe'  s  '  'true  associates. ' ' 


CHAPTER   IX 


Ocular  Paralyses  (Continued} 

Optical  Illusion. — Patients  sometimes  mention  that  the  lower 
of  the  two  images,  which  are  seen  when  any  depressor  muscle  is 
paralyzed,  appears  nearer  to  them  than  the  true  image.  As  bearing 
on  this,  Nagel  has  shown  that  a  ball  hanging  on  a  thread,  presented 
to  the  inspection  of  a  person  with  vertical  diplopia,  appears  as  two 
balls,  one  vertically  above  the  other  ;  while  the  same  ball  on  a  plate 
appears  as  two  balls  (and  of  course  two  plates),  one  in  front  of  the 
other.  This  clearly  indicates  the  nature  of  the  phenomenon  in  our 
patients.  They  estimate  the  nearness  of  the  false  image  with  reference 
to  a  horizontal  plane,  generally  \\-\zfloor,  which  corresponds  to  Nagel' s 
plate,  instead  of  estimating  it  with  reference  to  a  vertical  plane,  such 
as  the  wall,  which  would  answer  to  his  thread.  A  line  proceeding 
from  the  eye  through  the  lower  image  would,  of  course,  strike  the 
floor  at  a  nearer  point  than  a  similar  line  through  the  higher  image. 

The  illusion  can,  therefore,  be  dissipated  by  any  plan  which 
occupies  the  patient  with  the  wall  to  the  exclusion  of  the  floor  ;  as, 
for  instance,  by  placing  the  candle  or  test  object  against  the  wall  at 
a  sufficient  height  while  the  patient's  head  is  thrown  back  a  litfle 
(Landolt). 

If  the  false  image  also  appear  smaller  than  the  other,  it  is 
probably  only  because  it  is  thought  to  be  nearer. 

How  to  Transfer  Charts  to  Opposite  Eyes.— It  is  usual  in  text- 
books to  give  the  charts  of  ocular  paralyses  for  one  eye  only,  since 
it  is  supposed  to  be  so  easy  for  the  reader  to  mentally  transfer  each 
feature  of  the  chart  from  right  to  left,  or  vice  versa.  And  truly,  it 
is  easy  ;  but  for  anyone  in  haste,  or  not  accustomed  to  the  subject, 
to  make  a  mistake  in  so  doing,  is,  perhaps,  easier  still. 

To  overcome  the  difficulty  in  the  case  of  beginners,  I  would 
propose  one  of  the  following  simple  expedients,  any  one  of  which 
will  suffice  : 

(1)  Prick  the  chart  through  to  the  back  of  the  page  ;    this 
affords  the  required  transference.      Or, 

(2)  Hold  the  chart  before  a  looking-glass,  its  image  in  which 
affords  what  is  needed.      Or, 

(168) 


Ocular  Paralyses 


169 


(3)  Imagine  the  chart,  by  closing  and  reopening  the  book,  to 
have  become  transferred  to  the  opposite  page  ;  or  actually  trace  the 
chart  over  with  copying  ink  and  transfer  it  to  a  thin  sheet  of  paper 
gummed  into  the  book.      Of, 

(4)  To  obtain  the  chart  for  any  uncharted  superior  or  inferior 
rectus,  turn  the  chart  for  the  antagonistic  rectus  in  the  other  eye 
upside  down.      For  instance,  the  right  superior  rectus  is  charted  by 
inverting  the  chart  for  the  left  inferior  rectus.      Exactly  the  same 
procedure  avails  also  for  the  obliques.      The  chart  for  any  superior 
muscle  need  only  be  folded  from  above  downwards,  and  transferred 
to  give  the  chart  of  the  corresponding  inferior  muscle  of  the  same 
eye.     The  best  plan  of  all,  however,  js  to  possess  a  chart  of  every 
muscle,  to  save  time,  and  this  I  have  given. 

Let   us   now    consider   the    individual    paralyses.      It   will    be 
noticed  to  be  roughly  true  that 

1.  The  primary  deviation  (i.  e. ,  that  of  the  paralyzed  eye)  is 
opposite  to  the  direction  in  which  the  healthy  muscle  turns  the  eye. 

2.  The  remaining  displacements  are  all  in  the  same  direction 
as  that  in  which  the  healthy  muscle  turns  the  eye. 

PARALYSIS  OF  RIGHT  EXTERNAL  RECTUS 

This    muscle    turns    the    cornea  to  the  right,   therefore   there 
result : 

i.   Primary  Deviation  (of  paralyzed  eye) — To  the  patient's  left. 


Fig.  57 


170 


Tests  and  Sludies  of  the  Ocular  Muscles 


2.  Face  looks 

Defect  in  Motion  of  Eye 

Secondary  Deviation  (i.  e.,  of  sound  eye) 

Malprojection 

Maximum  Diplopia 

False  Image  Displaced 


To  the 
patient's  right. 


Diplopia. — Horizontal,  homonymous,  increasing  on  looking  to 
the  right,  and  also  with  recession  of  the  test  object.  Greater,  too, 
on  looking  downwards  ;  less  on  looking  up. 

PARALYSIS  OF  LEFT  EXTERNAL  RECTUS 

The  muscle  turns  the  cornea  to  the  left,  therefore  there  result : 

1.  Primary  Deviation — To  the  patient's  right. 

2.  Everything  else — To  the  patient's  left. 


Fig.  58 

Diplopia. — Horizontal,  homonymous,  increasing  on  looking  to 
the  left,  and  also  with  recession  of  the  test  object.  Greater,  too,  on 
looking  downwards  ;  less  on  looking  up. 

QUALIFICATIONS  IN  PARALYSIS  OF  THE  EXTERNAL  RECTI 

Two  unimportant  refinements,  one  of  which  has  already  been 
partly  indicated,  require  notice  in  paralysis  of  either  external  rectus. 


Ocular  Paralyses  171 

First  Qualification. — The  horizontal  separation  of  the  images  is  apt  to 
increase  on  looking  downwards,  and  lessen  on  looking  upwards.  This  is 
due  to  the  habit  of  the  converging  center,  for  looking  downwards  is  gen- 
erally associated  with  convergence,  and  looking  upwards  with  parallelism 
of  the  visual  axes. 

Second  Qualification. — On  looking  up  and  out,  the  inferior  oblique  of 
the  paralyzed  eye  ;  and  on  looking  down  and  out,  the  superior  oblique,  lose 
their  usual  torsional  purchase  on  the  paralyzed  eye,  because  its  visual  line 
keeps  nearer  their  muscular  plane.  The  purchase  which  they  lose  torsionally, 
however,  they  gain  vertically,  so  that  the  weak  eye  is  really  superducted  and 
subducted  slightly  more  than  the  sound  one,  when  the  latter  looks  respec- 
tively up  and  out,  and  down  and  out. 

In  consequence  of  this,  the  false  image,  on  looking  up  and  out,  is  lower 
than  the  true  and  extorted,  and  on  looking  down  and  out  is  higher  than  the 
true,  and  intorted.  It  will  be  remembered  that,  under  physiological  condi- 
tions, there  is  "false  torsion"  on  looking  in  these  directions,  but  the  effect 
of  the  paralysis  is  to  abolish  it,  since  it  disables  the  eye  from  turning  out, 
and  false  torsion  is  absent  from  the  simple  vertical  motions  of  the  eye. 
Indeed,  paralysis  of  the  external  rectus  affords  a  beautiful  demonstration  of 
the  existence  in  health  of  false  torsion,  since  the  mind,  accustomed  to 
reckon  for  it,  projects  the  false  image  precisely  in  its  direction.  The  lack  of 
torsion  in  the  eye  causes  exaggerated  torsion  of  the  image,  eye  and  image 
being  always  opposite  in  their  deviations. 

Putting  these  two  qualifications  together,  we  find  it  necessary  to  slightly 
modify  the  preceding  statements. 

Primary  Deviation. — Inwards  :  inwards  and  slightly  upwards  in  the 
upper  outer  parts  of  the  field  ;  inwards  and  slightly  downwards  in  the  lower 
outer  parts. 

Face  Looks — To  the  right ;  perhaps  also  slightly  upwards. 

Defective  ^lavement. — To  right.  More  so  when  the  eyes  look  down- 
ward. 

Secondary  Deviation.       \      In  the  upper    ]    f    Outwards 
outer  parts  of  >  •<   and  slightly 

Malprojection.  the  field.        )    (  downwards. 

[  In  the  lower  ]  f  Outwards 
False  Image  Displaced.  outer  parts  of  \\  and  slightly 

J  the  field.  }  (  upwards. 
Maximum  Diplopia. — On  looking  down  and  out. 

PARALYSIS  OF  RIGHT  INTERNAL  RECTUS 

This  muscle  turns  the  cornea  to  the  left,  therefore  there  result : 

1 .  Primary  Deviation  (of  paralyzed  eye) — To  the  pat'ent'  s  right. 

2.  Face  looks  1 
Defect  in  Motion  of  Eye 


Secondary  Deviation  (of  sound  eye) 
Malprojection 
Maximum  Diplopia 
False  Image  Displaced 


io  tne 


patient's  left. 


172 


Tests  and  Studies  of  the  Ocular  Muscles 


Diplopia. — Horizontal,  crossed,  increasing  on  looking  to  the 
left,  and  also  with  approach  of  the  test  object.  Greater,  too,  on 
looking  up  ;  less  on  looking  down. 


Fig.  59 


PARALYSIS  OF  LEFT  INTERNAL  RECTUS 

This  muscle  turns  the  cornea  to  the  right,  therefore  there  result 

1.  Primary  Deviation— To  the  patient's  left. 

2.  Everything  else— To  the  patient's  right. 


Fig.  6O 


Ocular  Paralyses  173 

Diplopia. — Horizontal,  crossed,  increasing  on  looking  to  the 
right  and  also  with  approach  of  the  test  object.  Greater,  too,  on 
looking  up  ;  less  on  looking  down. 

QUALIFICATIONS  IN  PARALYSIS  OF  THE  INTERNAL  RECTI 

Two  unimportant  refinements  (akin  to  those  for  the  external 
recti)  require  notice. 

First  Qualification. — The  horizontal  separation  of  the  images  is  apt  to 
increase  on  looking  upwards  and  lessen  on  looking  downwards,  due,  as  in 
the  case  of  the  external  recti  (q.  v.),  to  the  habit  of  the  converging  center. 

Second  Qualification. — On  looking  up  and  in,  the  superior  rectus  of  the 
paralyzed  eye  ;  and  on  looking  down  and  in,  the  inferior  rectus  of  the  same 
eye,  lose  torsional  purchase  over  the  eye  from  the  fact  that  their  muscular 
plane  forms  a  smaller  angle  with  its  visual  axis  than  when  adduction  is 
efficient.  Hence,  on  looking  up  and  in,  the  false  image  should  theoretically 
be  lower  than  the  true  and  intorted ;  and  on  looking  down  and  in,  it  should 
be  higher  than  the  true  and  extorted.  Absolutely  isolated  paralysis  of  the 
internal  recti  are,  however,  so  rare  that  this  could  scarcely  have  been  dis- 
covered from  actual  experiment.  It  is  chiefly  from  the  analogy  of  the 
external  recti  that  it  is  assumed  to  occur.  It  will  be  seen  represented  in 
the  charts. 

PARALYSIS  OF  RIGHT  SUPERIOR  RECTUS 


Fig.  61 


This  muscle  turns  the  cornea  upwards,  and  somewhat  inwards, 
with  intorsion.  Its  power  as  a  superductor  is  greatest  during 
vision  to  the  right.  It  is,  therefore,  a  "  dextral  superductor." 


Jests  and  Studies  of  the  Ocular  Muscles 

Its  power,  on  the  other  hand,  as  adductor  and  intorter  is 
greatest  during  vision  to  the  left.  Therefore,  there  result  from  its 

paralysis  : 

1.  Primary  Deviation  (/.  e.,  of  paralyzed  eye) — Downwards ; 
and,  in  the  upper  half  of  the  field,  outwards ;  with  extorsion. 

2.  Face  looks —  Up  and  to  the  right. 

Defect  in  Motion  of  Eye — Upwards;  most  marked  when  the 

eyes  are  also  turned  to  the  right. 
Secondary  Deviation — (i.  e.,  of  sound  eye) — Upwards,  and 

slightly  to  the  left,  probably  with  a  little  laevotorsion. 
Malprojection —  Upwards,  and  slightly  to  the  left. 
Maximum  Diplopia — On  looking  up  and  to  the  right. 
False  Image  Displaced — Upwards  and    slightly  to   the  left, 

with  laevotorsion. 

Diplopia. — Vertical  diplopia,  increasing  on  looking  up,  and 
especially  up  and  to  the  right  ;  crossed  and  torsional  diplopia 
increasing  on  looking  up  and  to  the  left.  The  nature  of  the 
diplopia  during  the  primary  position  of  the  sound  eye  should  be 
carefully  investigated,  and  the  equilibrium  should  be  examined 
(by  the  rod  test,  e.  g.~)  in  the  non-diplopic  half  of  the  field. 
This  remark  applies  equally  to  all  the  succeeding  paralyses. 

PARALYSIS  OF  LEFT  SUPERIOR  RECTUS 
Same  as  the  last,  with  substitution  of  "left"   for  right,  and 


LEFT   SUPERIOR  RECTUS 


Fig. 


Ocular  Paralyses 


175 


vice  versa.      It  turns  the  cornea  up  and  in,  with  inward  torsion,  and 
is  a  laeval  elevator.      (Fig.  62.) 

PARALYSIS  OF  RIGHT  INFERIOR  RECTUS 
This    muscle    turns    the    cornea   downwards    and    somewhat 
inwards,   with  extorsion.      Its    power   as  a    subductor    is   greatest 


fig.  63 

during  vision  down  and  to  the  right.  It  is,  therefore,  a  "dextral 
subductor."  Its  power,  on  the  other  hand,  as  an  adductor  and 
extorter  is  greatest  during  vision  down  and  to  the  left.  There 
result  from  its  paralysis 

1.  Primary  Deviation  (/.  e.,  of  paralyzed  eye) — Upwards  and, 
in  the  upper  half  of  the  field,  outwards ;  with  intorsion. 

2.  Face  looks — Down  and  to  the  right. 

Defect  in  Motion  of  Eye—  Downwards,   most    marked    when 

the  eyes  are  also  turned  to  the  right. 
Secondary    Deviation    (i.   <?.,    of    sound    eye) — Downwards. 

slightly  to  left,  with  dextrotorsion  ? 
Malprojection — Downwards,  slightly  to  left. 
Maximum  Diplopia — On  looking  down  and  to  the  right. 
False  Image — Downwards  and  slightly  to  left. 
Diplopia. — Vertical  diplopia,  greatest  on  looking  down  and  to 
the  right  ;  crossed  torsional  diplopia,  greatest  on  looking  down  and 
to  the  left. 


J?6  /;.,/,  and  Studies  cf  the  Ocular  Muscles 

PARALYSIS  OF  LEFT  INFERIOR  RECTUS 


Fig.  64 


Same  as  the  last,  with  the  substitution  of  right  for  left,  and 
vice   versa   throughout.      It   turns    the  cornea  down  and  m,   WltJ 
extortion,  and  is  a  "  keval  depressor." 

PARALYSIS  OF  RIGHT  SUPERIOR  OBLIQUE 


Fig.  65 


This  muscle  turns  the  cornea  downwards  and  outwards^  with 
intorsion.      Its  power  as  a  depressor  is  greatest  during  vision  to  the 


Ocular  Paralyses 


177 


left  ;  it  is,  therefore,  a  "  laeval  depressor."  Its  power,  on  the  other 
hand,  as  an  intorter  and  abductor  is  greatest  during  vision  to  the 
right,  because  then  its  muscular  plane  forms  the  greatest  angle  with 
the  visual  axis.  There  result  from  its  paralysis  : 

1.  Primary  Deviation  (/.  <?.,  of  paralyzed  eye) — Upwards  and, 
in  the  lower  half  of  the  field,  slightly  to  the  left ;  with  cxtorsion. 

2.  Face  looks — Down  and  to  the  left. 

Defective  Motion  of  Eye — Downwards,  especially  when  vision 

is  directed  down  and  to  the  left. 
Secondary  Deviation  (i.  e.,  of  sound  eye) — Downwards  and 

slightly  to  the  right,  and  laevotorted. 
Malprojection — Downwards  and  slightly  to  the  right. 
Maximum  Diplopia — Downwards  and  to  left. 
False  Image  Displaced — Downwards,    slightly  to  the  right, 

and  laevotorted. 

Diplopia. — Vertical  diplopia,  greatest  on  looking  down  and  to 
the  left  ;  torsional  diplopia  greatest  on  looking  down  and  to  the 
right  ;  generally  homonymous  diplopia  greatest  on  looking  down 
and  to  the  right,  but  sometimes  crossed. 

PARALYSIS  OF  LEFT  SUPERIOR  OBLIQUE 


LEFT  SUPERIOR  .OBLIQUE 


Fig.  60 


Same  as  the  last,  substituting  right  for  left,  and  vice  versa 
throughout.  It  turns  the  cornea  down  and  out,  with  intorsion,  and 
is  a  "  dextral  depressor." 


178  Tests  and  Studies  of  the  Ocular  Muscles 

PARALYSIS  OF  RIGHT  INFERIOR  OBLIQUE 

This  muscle  turns  the  cornea  up  and  out,  with  extorsion.  Its 
power  as  a  superductor  is  greatest  during  vision  to  the  left  ;  it  is, 
therefore,  a  "laeval  elevator."  As  an  extorter  and  abductor,  on 
the  other  hand,  its  power  is  greatest  during  vision  to  the  right, 
when  its  muscular  plane  forms  the  greatest  angle  with  the  visual 
line.  There  result  from  its  paralysis  : 

1.  Primary  Deviation  (of   paralyzed  eye) — Downwards  and, 
in  the  upper  half  of  the  field,  inwards  ;  with  intorsion. 

2.  Face  looks —  Up  and  to  the  left. 

Defect  in  Motion  of  Eye —  Upwards,   especially  when  vision 

is  also  directed  to  the  left. 
Secondary   Deviation   (i.   e.,  of  sound  eye) — Upwards  and 

slightly  to  the  right,  with  dextrotorsion. 
Malprojection —  Upwards  and  generally  to  the  right. 
Maximum  Diplopia — On  looking  up  and  to  the  left. 
False  Ima^e  Displaced —  Upwards  and  generally  to  the  right, 

with  dextrotorsion  always. 


Fig.  67 


iMplOpia.— Vertical  diplopia,  greatest  on  looking  up  and  to  the 
;  torsional  diplopia  greatest  on  looking  up  and  to  the  right  ; 

generally  homonymous  diplopia  greatest  on  looking  up  and  to  the 

right,  but  sometimes  crossed. 


Ocular  Paralyses 


179 


PARALYSIS  OF  LEFT  INFERIOR  OBLIQUE 

Same  as  the  last,  with  substitution  of  right  for  left,  and  vice 
versa  throughout  the  description.  It  turns  the  cornea  2*p  and  out, 
with  exiorsion,  and  is  a  "  dextral  elevator." 


Fig.  68 

There  are  several  special  points  of  interest  about  paralysis  of 
the  inferior  oblique. 

(a)  It  is  a  powerful  elevator.  Mauthner's  observation  that 
the  vertical  diplopia  from  paralysis  of  the  inferior  oblique  is  greater 
than  that  of  the  superior  oblique,  has  been  referred  to.* 

(£)  Traumatic  paralysis  of  this  muscle  from  direct  injury  has 
been  recorded  by  Noyes,  Berry,  etc. 

(c}  Since  the  short  root  of  the  lenticular  ganglion  is  derived 
from  the  branch  of  the  third  nerve,  which  supplies  the  inferior 
oblique  and  conveys  the  motor  fibres  for  the  ciliary  and  sphincter 
iridis  muscles,  no  posterior  orbital  paralysis  of  the  nerve  to  the 
inferior  oblique  could  occur  without  involving  also  the  intrinsic 
muscles  of  the  eye. 

(</)  It  is  highly  probable,  though  not  absolutely  proved,  that 
the  nucleus  for  the  inferior  oblique  lies  on  the  opposite  side  of  the 
median  plane,  necessitating  a  decussation  of  its  fibers  of  origin 
analogous  to  that  of  the  superior  oblique.  (Indeed,  we  may  say 

*  It  is  interesting  to  observe,  as  perhaps  related  to  this  fact,  that  the  insertions  of  tho 
two  obliques  are  not  parallel  to  each  other,  hut  form  a  V,  in  such  a  way  as  to  indicate  that  the 
inferior  oblique  has  the  greater  vertical  purchase  of  the  two. 


i8o  Tests  and  Studies  of  the  Ocular  Muscles 

that  the  obliques  are  oblique  in  every  way.  They  are  innervated 
from  the  opposite  side  of  the  brain  ;  their  maximum  diplopia  is  on 
the  side  opposite  to  the  affected  eye,  and  is  in  every  way  oppositely 
named,  being  below  when  the  oblique  is  above,  and  vice  versa.) 
In  unilateral  nuclear  paralysis  of  the  third  nerve  it  is  of  interest  to 
look  for  paresis  of  the  opposite  inferior  oblique,  with  escape  of  the 
inftrior  oblique  on  the  paralyzed  side.  The  evidence  would  be  : 
(i)  Defective  upward  movement  of  the  good  eye,  especially  on 
looking  up  and  in.  (2)  Torsion  of  the  good  eye  on  looking  up 
and  out,  causing  extorsion  of  the  image  produced  with  the  good 
eye  by  the  glass-rod  test.  (3)  Some  upward  movement  of  the 
lame  eye,  especially  on  looking  up  and  in.  (4)  Extorsion  of  the 
lame  eye  (with  intorsion  of  its  image)  on  looking  up  and  out. 

PARALYSIS  OF  THE  THIRD  NERVE 

May  be  nuclear,  sub-nuclear  or  peripheral  ;  partial  or  complete. 

The  extra-ocular  muscles  supplied  by  this  nerve  may  be  com- 
pletely paralyzed  (the  superior,  internal  and  inferior  rectus,  and 
inferior  oblique,  with  the  levator  palpebrae)  without  any  involve- 
ment of  the  intra-ocular  muscles  (the  sphincter  iridis  and  ciliary 
muscle)  ;  or  vice  versa.  Such  cases  are  almost  certainly  nuclear. 
On  the  other  hand,  partial  involvement  of  some  of  the  extra-ocular 
muscles,  with  only  a  partial  participation  of  the  intra-ocular,  are 
almost  certainly  not  nuclear. 

Signs. — The  outstanding  features  of  a  complete  paralysis  of  the 
third  nerve  are  : 

(1)  Ptosis,  with  vicarious  contraction  of  the  occipito-frontalis 
on  that  side. 

(2)  Abduction  of  the  eye,  increasing  as  time  goes  on,  com- 
bined with  slight  subduction. 

(3)  Immobility  of  the  eye  in  all  directions  except  outwards, 
and  outwards  and  downwards. 

(4)  Semi-dilated*  pupil,  immobile  to  light  and  to  consensual 
influence. 

(5)  Loss  of  accommodation. 

(6)  The  face  generally  looks  straight  forward,  since  it  is  not 
often  that  an  attempt  is  made  to  overcome  the  diplopia.      When 
such  attempt  is  made,  however,  if  the  right  eye  be  paralyzed,  the 

*  As  time  wears  on,  the  pupil  becomes  more  fully  dilated. 


Ocular  Paralyses 


181 


face  looks  to  the  left  and  slightly  upwards  ;  if  the  left  eye,  the  face 
looks  to  the  right  and  slightly  upwards. 

Pathetic  Nerve. — The  most  interesting  fact  to  elicit  is  whether 
or  not  the  fourth  nerve  is  involved.  The  usual  way  of  testing  this 
is  to  make  the  sound  eye  follow  the  finger  from  above  downwards, 
while  the  affected  eye  is  closely  watched  to  see  if  the  cornea  rotates 
about  its  own  pupil  like  a  wheel,  under  the  influence  of  the  superior 
oblique.  That  muscle  being  an  intorter,  rotates  the  upper  part  of 


fig.  69 

the  cornea  inwards  at  the  same  time  that  it  displaces  the  whole 
cornea  down  and  out. 

What  I  think  a  better  plan  is  to  hold  one  forefinger  ?bove  and 
the  other  below  the  level  of  the  patient's  face,  and  make  him  look 
quickly  from  one  to  the  other.  It  is  always  easier  to  detect  a  quick 
torsion  than  a  slow  one.  If  the  superior  oblique  be  paralyzed  as 
well  as  the  whole  third  nerve,  the  descent  of  the  good  eye  pro- 
duces no  movement  in  the  affected  one. 

Subjective  measurement  of  the  torsion  by  glass  rods  enables  a 
more  delicate  test  to  be  made  than  any  objective  test  affords.  In 
slight  third-nerve  pareses,  raising  the  face  will  cause  extorsion  of 
the  light  streak  if  the  fourth  nerve  be  intact.  In  complete  paralysis 
this  test  can  be  supplemented  by  making  the  patient  keep  his  head 
still,  cover  his  good  eye  with  his  hand  and  direct  it  alternately  to 
the  ceiling  and  the  floor,  while  the  glass  rods  are  held  before  the 
paralyzed  eye ;  with  the  ophthalmoscope,  too  (as  Hirschberg, 


182 


Tests  and  Studies  of  the  Ocular  Muscles 


according  to  Asher,  first  pointed  out),  torsional  movement  of  the 
retinal  vessels  is  well  made  out. 

In  estimating  the  amount  of  ptosis  we  must  take  into  account 
that  the  overflow  of  nervous  force  into  the  occipito-frontalis  muscle 
(on  the  same  principle  as  the  "secondary  deviation"  in  the  case 
of  the  ocular  muscles),  when  an  effort  is  made  to  raise  a  drooping 
lid,  may  cause  a  slight  false  elevation  of  the  lid  even  when  the 
levator  is  completely  paralyzed.  To  guard  against  this  fallacy 
the  eyebrow  has  been  recommended  to  be  firmly  fixed  by  pres- 
sure against  the  frontal  bone  before  testing  the  power  to  raise  the 
upper  lid. 


Fig.  70 


Figs.  69  and  70  show,  very  diagrammatically,  however,  the 
nature  of  the  diplopia  in  cases  of  third-nerve  paralysis,  after  A.  Pick. 

MEASUREMENT  OF  OCULAR  PARALYSES 

Measured  Paralyses.— It  is  always  a  good  plan,  when  time 
permits,  to  take  as  accurate  measurements  as  we  can  of  the  diplopia 
in  all  nine  areas  of  the  motor  field.  By  repeating  such  measure- 
ments from  time  to  time  an  excellent  idea  of  the  progress  of  the 
case  is  formed,  and  difficulties  in  the  diagnosis  may  be  cleared  up. 
Actual  measurements  of  the  motility  of  the  eye,  though  useful,  are 
less  reliable,  since  here  much  depends  on  the  varying  will-power  of 
the  patient  and  the  amount  of  effort  made. 

Mode  Of  Measurement.— The  glass-rod  test,  used  in  conjunc- 
tion with  tangent  scales  adjusted  on  the  wall,  makes  such  measure- 


Ocular  Paralyses 


183 


ments  comparatively  easy.  A  vertical  scale  should  be  suspended 
across  the  middle  of  the  horizontal  scale  and  a  small  but  brilliant 
source  of  light  be  placed  in  front  of  their  intersection. 


Fig.  71 

Tangent  Scales  in  degrees  for  five  meters.    The  line  S  8 represents  the  streak  of  light 
produced  by  the  glass  rods. 

The  patient,  standing  five  meters  away,  should  have  his  head 
Dosed  in  nine  different  attitudes,  corresponding,  as  regards  the 
eyes,  to  the  nine  motor  areas,  for  the  eyes  themselves  are  to  be 
diiected  towards  the  flame  in  front  of  the  intersecting  scales. 
A  mounted  series  of  rods  being  held,  first  horizontally  and  then 
vertically,  before  one  eye,  it  is  easy  for  the  patient  to  read  off  the 
vertical  and  horizontal  elements  of  the  diplopia,  in  turn,  for  each 
attitude  of  the  head. 

Entries. — Fig.  72  shows  a  convenient  way 
of  noting  down  the  results.  Placing  a  dot  in  the 
center  of  each  square  to  represent  the  true  image, 
insert  another  dot  to  represent  the  false  image, 
and  numerate  its  horizontal  and  vertical  elements 
as  represented  by  the  thinner  lines,  somewhat 
on  the  principle  of  rectangular  co-ordinates, 
only  in  degrees  instead  of  in  linear  units.  If  de- 
sired, even  the  degree  of  torsion  can  also  be 
recorded,  by  finding  what  inclination  must  be 
imparted  to  the  rods  to  make  the  streak  of  light  appear  vertical  or  horizontal. 

The  Examination  of  Torsion  Facilitated.— The  difficulty  which 
has  hitherto  attended  the  investigation  of  the  torsional  elements 
of  paralysis  is  very  much  lessened  by  the  rod,  for  the  patient's 


Fig.  72 


1 84  Tests  and  Studies  of  the  Ocular  Muscles 

power  of  observation  can  be  analyzed  by  tilting  the  streak  slightly 
to  one  or  other  side,  asking  the  patient  to  describe  the  inclination. 
In  this  way  we  learn  just  how  much  confidence  to  repose  in  his 
statements. 

The  record  in  Fig.  72  means  that  in  the  right  superior  area  of 
the  motor  field  the  false  image  is  displaced  6°  to  the  left  (laevo- 
ducted  6°),  and  9°  upwards  (superducted  9°)  above  the  true 
image,  and  has  a  laevotorsion  of  8°. 

A  very  good  plan  is  to  use  mathematical  paper,  which  is 
marked  with  very  small  squares,  and  to  esteem  each  square 
a  degree.  If  more  space  is  needed,  the  true  image  need 
not,  of  course,  be  represented  as  in  the  center  of  the  square, 
but  can,  for  convenience,  be  displaced  to  any  suitable  corner, 
provided  it  be  notified  that  the  center  of  the  square  is  the 
proper  place  for  it,  since  it  indicates  the  direction  of  vision  of 
the  sound  eye. 

The  tangent  scales  described  serve  not  only  for  paralyses, 
but  also  for  heterophoria,  for  objective  strabismometry  and  for 
prismetry. 

As  regards  the  horizontal  displacement  of  a  false  image,  it  is 
well  known  that  in  paralysis  of  obliques  a  pre-existing  latent  diver- 
gence (exophoria)  may  be  set  free  (i.  e.,  cease  to  be  latent  and 
become  manifest)  in  the  diplopic  area  of  the  field,  and  may  so 
complicate  the  case  as  to  convert  the  typical  "homonymous" 
diplopia  into  "crossed." 

A  similar  complication  may  occur,  though  less  likely  to  do  so, 
in  paralyses  of  the  superior  and  inferior  recti,  where  there  is  a 
possibility  of  a  pre-existing  latent  convergence  (esophoria)  con- 
verting the  typical  "  crossed  "  dipiopia  into  '•'  homonymous." 

Further,  with  regard  to  torsion  of  the  false  image,  Mauthner 
called  particular  attention  to  the  untrustworthiness  of  the  answers 
given  by  patients  on  this  point,  though  it  is  true  that  by  using  the 
glass-rod  test  the  difficulty  is  lessened,  the  tilting  of  a  long  streak 
of  light  being  easily  observed  when  compared  with  a  vertical  line 
on  the  wall. 

In  addition  to  this,  I  imagine  that  cases  occur  in  which, 
however  accurate  the  patient  may  be  in  describing  the  false  image, 
the  paralytic  torsion  is  (at  least  in  parts  of  the  field)  overborne  by 
a  greater  pre-existing  latent  torsion  exerted  in  the  opposite  sense 
so  as  to  afford  a  misleading  index. 


Ocular  Paralyses  185 

In  confirmation  of  this,  it  may  be  adduced  that,  on  incidentally 
testing  a  refraction  case,  I  found  as  much  as  10°  of  latent  extorsion 
associated  with  distant  vision,  which  increased  to  20°  with  vision 
for  twenty  inches.  Such  cases  must  be  rather  rare,  but  should 
they  at  any  time  subsequently  develop  paralysis  of  an  extorting 
muscle  (inferior  rectus  or  inferior  oblique),  the  latent  condition 
would  greatly  complicate  the  torsion  of  the  false  image.  The 
only  way  to  detect  it  would  be  to  study  the  torsion  in  all  parts  of 
the  field. 

In  another  patient  (complicated,  however,  with  inconcomitant 
hyperphoria,  so  that  it  was  not  so  remarkable)  I  also  found  10°  of 
latent  torsion  in  distant  vision.  Both  were  men,  and  had  persistent 
tendency  to  headaches. 

All  these  considerations  accentuate  the  value  of  Mauthner's 
advice  to  pay  attention  only  to  the  vertical  element  of  the  diplopia 
in  paralysis  of  the  torsion-producing  muscles,  at  least  for  the  rough 
primary  diagnosis. 

Mnemonics. — The  mnemonic  I  would  suggest  as  least  mislead- 
ing consists  in  so  naming  the  areas  of  the  field  of  diplopia  that  the 
area  of  greatest  vertical  diplopia  shall  be  the  namesake  either  of 
the  affected  muscle  or  of  its  true  associate  in  the  other  eye.  No 
diagram  is  needed  for  this  and,  happily,  no  arbitrary  nomenclature 
of  the  areas  is  required,  since  their  own  proper  names  afford  just 
what  is  wanted. 

There  is,  therefore,  no  effort  of  memory,  each  area  having  its 
natural  name  from  the  patient's  point  of  view. 

Thus  the  right  superior  area  is  that  which  lies  in  the  upper 
part  of  the  field,  to  the  patient's  right.  Maximum  vertical  diplopia 
found  in  this  situation  means  paralysis  of  the  namesake  muscle,  the 
right  superior  rectus,  or  else  of  its  true  associate  in  the  other  eye, 
the  left  inferior  oblique.  It  Is  easy  to  settle  between  these  two  by 
finding  to  which  eye  the  false  image  belongs. 

The  true  associates  can  always  be  borne  in  mind  by  remember- 
ing that  their  names  are  the  most  contrary  possible.* 

In  short,  having  found  the  area  of  greatest  vertical  diplopia, 
the  paralyzed  muscle  is — either  the  same-named  rectus  or  the  cross- 
named  oblique. 

There  are  two  plans  of  recording  ocular  paralyses  at  present  : 
in  one  of  these  the  surgeon  transfers  himself  to  the  patient's  position 

*For  example,  left  inferior  oblique  is,  in  every  term,  opposite  to  right  superior  rectus. 


1 80  Tests  and  Studies  of  the  Ocular  Muscles 

and     looks    at    the    motor    field    from    the     patient's    point    of 
view,  as   in    Fig.   73,    where   the   small    print   is   borrowed    from 

Eugene  Pick. 

In  the  other  plan  the  surgeon  selects  the  easier  task  of  imagin- 
ing himself  where  he  really  is,  and  looks  at  the  motor  field  from  his 
own  side.  It  is  as  though  a  grating  or  a  window  frame  were  sus- 
pended in  the  air  between  the  patient  and  himself  the  areas  being 


THE  PATIENT  LOOKS 


R. 


Upwards  to  the  Wt. 
LEFT 

SUPERIOR 
AREA. 

Upwards. 

SUPERIOR 
.MEDIAN 
AREA. 

Upwards  to  the  right. 

RIGHT 
SUPERIOR 
AREA. 

To  the  left. 

Straight  ahead. 

To  the  right, 

LEFT 
EXTERNAL 
AREA. 

PRIMARY 
AREA. 

RIGHT 
EXTERNAL 
AREA. 

Downwards  to  the  left. 

Downwards. 

Downwards  to  the  right. 

LEFT 
INFERIOR 
AREA. 

INFERIOR 
MEDIAN 
AREA. 

RIGHT 
INFERIOR 
AREA. 

Ki«.  73 
Author's  Mnemouic  Motor  Field. 

named  by  the  patient,  but  looked  at  by  the  surgeon.  These  two 
plans  correspond  precisely  to  the  two  methods  in  vogue  for  record- 
ing the  lenses  in  a  trial  frame.* 

The  "namesake"  mnemonic  just  described  answers  in  both 
modes  of  entry,  since,  in  each,  it  is  the  patient  who  names  the  areas. 
Though  I  have  hitherto  used  the  second  method  the  first  seems  in 
most  common  use,  and  in  this  work  it  has  been  adopted.  For 
confirmatory  evidences  we  need  only  remember  the  physiological 
action  of  the  impeached  muscle  and  that  the  image  is  displaced, 
when  the  eye  is  paralyzed,  in  every  way  like  the  eye  would  be 
displaced  if  it  were  sound. 

*The  Inconvenience  often  experienced  in  translating  one  mode  Into  another  mav  be  over- 

•  by  considering  that  one  mode  of  entry  looked  at  in  a  looking  glass,  or  transferred  by 

copvniK  ink  to  a  of  paper,  or  pricked  through  to  the  buck  of  the  paper  yields  the  other 


Ocular  Paralyses  187 

Conjugate  Deviations  and  Paralyses. — In  this  group  are 
included  only  those  cases  in  which  one  or  more  of  the  conjugate 
innervations  are  affected,  and  never  any  case  in  which  associated 
muscles  may,  as  a  coincidence,  happen  to  be  simultaneously 
paralyzed.  We  have  already  seen  that  probably  all  the  co-ordi- 
nating innervations  of  the  eyes  are  conjugate. 

Diplopia  is  not  Characteristic.— All  the  paralyses  of  parallel 
motion  have  this  feature  in  common  :  that  diplopia  is  either  absent 
or  inconsequential. 

Homonymous  Restriction  of  the  Motor  Field. — Only  by  undue 
limitation  of  the  parallel  motions  of  both  eyes  in  one  or  more  direc- 
tions can  defects  in  these  motions  be  diagnosed. 

Convergence  Defects. — It  is  most  likely  that  convergence  de- 
fects divide  themselves  into — 

(1)  Paralyses   of    convergence    of    nuclear    or    supranuclear 
origin  ; 

(2)  Insufficiency  of  convergence,   either  latent,  as   in    "exo- 
phoria,"  or  too  great  to  be  suppressed,  as  in  "concomitant  diver- 
gent squint"  ;  and 

(3)  Absence  of    convergence    effort,   from  want  of   use   when 
binocular  fixation  has  been  lacking  for  a  number  of  years. 

Conjugate  Elevation-Paralysis  means  loss  of  the  power  of 
elevating  the  eyes.  Numerous  cases  have  been  recorded.  The 
more  evanescent  the  paralysis,  the  more  likely  is  its  seat  to  be  in 
the  cortex.  It  may  or  may  not  be  accompanied  with  ptosis. 

Spurious  Variety. — True  conjugate  elevation-paralysis,  in  which 
the  two  eyes  are  equally  concerned,  may  be  simulated  occasionally 
by  imperfect  development  of  both  superior  recti,  but  is  then  nearly 
always  conjoined  with  more  or  less  of  congenital  bilateral  ptosis. 
It  is  distinguished  from  true  conjugate  paralysis  by  efficiency  of  the 
inferior  obliques,  with  characteristic  diplopia  accordingly,  in  the 
upper  'half  of  the  field,  the  two  images  becoming  crossed  and 
inclined  away  from  each  other. 

Complications. — When  elevation-paralysis  occurs  as  a  nuclear 
affection,  it  is  apt  to  be  associated  with  more  or  less  depression- 
paralysis,  also  with  loss  of  converging  power,  and  of  its  associated 
contraction  of  the  pupil.  In  these  cases  the  determination  of 
accommodative  power  too  should  not  be  omitted. 

Conjugate  Depression-Paralysis  rarely  occurs  alone.  It  is 
generally  associated  with  loss  of  parallel  upward  motion  as  well. 


,ss  Tests  and  Studies  of  the  Ocular  Muscles 

Both  have  been  completely  lost,  in  many  recorded  cases,  leaving 
the  horizontal  motions  quite  unaffected. 

Right-and-Left  Motion.— Conjugate  parallel  motions  to  the 
right  or  left  may,  for  convenience,  be  called  ' '  ranging  motions. 

Lost  by  Cortical  Disturbance.— They  may  be  lost  from  sudden 
lesions  in  the  cerebral  cortex,  causing,  for  a  few  days  or  weeks, 
inability  to  turn  the  eyes  away  from  the  side  of  the  lesion  ;  the  gradual 
restoration  of  power  is  due  to  the  opposite  hemisphere  taking  up 
the  lost  function,  which  happily  is  represented  in  both  hemispheres. 

By  Pontine  Disturbance. — If  the  lesion  be  in  the  pons,  the 
rule  is  reversed,  since  the  eyes  now  cannot  be  turned  towards  the 

side  of  the  lesion. 

Conjugate  Lateral  Deviation. — Passive  deviation  of  both  eyes 

to  the  right  or  to  the  left  (corresponding  to  the  "primary  devia- 
tion" of  an  ordinary  concomitant  squint)  results  (a)  from  a  cor- 
tical lesion  on  the  same  side  as  the  deviation,  or  (£)  from  a  pontine 
lesion  on  the  opposite  side. 

Lateral  Spasm. — Conjugate  spasm  is  away  from  the  side  of  a 
cortical  lesion  and  towards  a  pontine  lesion  ;  also  towards  the 
paralyzed  side  of  the  body,  the  limbs  of  which  are  rigid.  "These 
are  merely  an  extension  to  the  eyes  of  effects  of  the  disease  manifest 
in  the  limbs"  (Gowers).  Thus  irritative  lesion  of  the  posterior 
third  of  the  left  mid-frontal  convolution  may  cause  the  patient  to 
turn  his  eyes  and  his  head  to  the  right. 

Path  of  the  Parallel  Right-and-Left  Innervation.— The  track 
of  the  innervation  is  from  the  cortical  motor  center  to  the  corpora 
quadrigemina,  thence  to  the  opposite  side  of  the  pons  (and  prob- 
ably through  the  superior  olivary  body)  to  the  nucleus  of  the  sixth 
nerve,  where  it  divides  equally  into  two,  one  proceeding  along  the 
sixth  nerve  to  the  external  rectiis  of  the  same  side  ;  the  other 
to  the  nucleus  of  the  third  nerve  for  the  internal  rectus  of  the 
other  eye. 

Since  the  neurology  of  the  subject  does  not  properly,  however, 
lie  within  the  scope  of  this  work,  we  now  pass  on  to  consider  the 
tests  for  conjugate  paralyses. 

A— Rough  Tests. 

(i)  Finger  Field.  Make  the  patient  follow  the  finger  (or 
some  small  striking  test  object)  in  all  four  cardinal  directions,  as 
previously  described,  to  detect  any  defect  in  parallel  excursions 
upwards,  downwards,  to  right  or  to  left. 


Ocular  Paralyses  189 

(2)  Finger  Near- Point  of  Convergence.  Approach  tht-  finger 
steadily  towards  the  root  of  the  nose,  making  the  patient  evoke 
every  effort  to  fix  the  finger  nail,  till  one  eye  commences  to  deviate. 
Notice  at  the  same  time  if  there  is  any  contraction  of  the  pupil 
associated  with  this  convergence  effort. 

B — Precise  Tests. 

( i )  By  Perimeter  or  Tangent  Scale.  Place  the  patient' s 
face  in  the  perimeter  and  test  the  excursions  of  the  eyes  in  each  of 
the  four  cardinal  directions.  This  may  be  done  either  objectively 
or  subjectively. 

OBJECTIVELY. — For  the  former,  hold  a  lighted  taper  or  elec- 
tric lamp*  at  the  normal  limit  of  the  motor  field,  close  to  the  arm  of 
the  perimeter  and  screened  from  the  surgeon's  eye  by  a  very  small 
screen,  which  should  be  provided  with  a  notch  just  opposite  the 
flame.  The  patient's  head  should  be  immovably  fixed,  preferably 
by  gripping  something  with  his  teeth,  and  with  his  face  set  straight 
for  the  central  ivory  disk.  He  should  now  be  encouraged  to 
attempt,  as  strongly  as  possible,  to  look  in  the  direction  of  the 
flame,  while  the  surgeon  notes  by  the  corneal  reflection  whether  he 
actually  succeeds. 

If  both  eyes  are  tested  together,  the  patient  should  have  his  chin 
placed  centrally  and  be  encouraged  to  accommodate  for  the  dis- 
tance of  the  arc.f 

To  test  without  convergence  or  accommodation,  it  is  necessary 
to  test  each  eye  separately,  placing  each  in  the  center  of  the  peri- 
meter in  turn.  When  this  is  done,  the  greatest  care  should  be 
taken  not  to  let  the  patient  fix  anything  in  particular,  since  the 
convergence  which  accompanies  any  attempt  to  accommodate 
lessens  the  arc  of  abduction. 

SUBJECTIVELY. — Subjectively,  the  test  is  easier  to  make, 
though  not  quite  so  satisfactory.  A  strip  of  paper  with  all  the 
letters  of  the  alphabet  in  line  but  out  of  order  on  it,  large 
enough  to  be  easily  read  by  central  fixation  but  too  small 
for  peripheral  vision  (J  7  is  about  the  best,  size,  I  think)  should 
be  applied  to  the  concave  surface  of  the  arm  of  the  perimeter, 
while  the  patient  is  told  to  read  the  farthest  letter  he  can  dis- 
tinguish (Casey  Wood). 

*The  most  luxurious  illumination  is  to  have  a  small  electric  lamp  fixed  on  the  carrier  of 
the  perimeter. 

t  Instead  of  a  perimeter,  the  distant  tangent  scale  can  be  used,  an  assistant  moving  a  flame 
aloug  it,  while  the  patient's  head  is  held  at  a  meter's  distance  by  another  assistant. 


Tests  and  Studies  of  the  Ocular  Muscles 

An  instrument  for  measuring  the  field  of  fixation  of  the 
eye,  called  the  tropometer,  has  been  designed  by  Dr.  Stevens. 
Whether  it  presents  any  advantage  over  the  perimeter,  is 

doubtful.  .       , 

(2)  Precise  Tests  for  Convergence.— These  can  likewise  be 

made  both  objectively  and  subjectively. 

(a)  Manifest  Strabismus.—  When  a  convergence  paralyses 
becomes  so  great  that  actual  divergent  squint  exists,  it  can  be 


Fig.  74 

Steven's  Tropometer. 

objectively  measured  either  by  Priestley  Smith's  tape  method  or  by 
those  described  in  Chapter  VII,  and  subjectively  by  the  rod  test 
and  tangent  scale. 

(£)  Convergence- effort. — Besides  measuring  the  squint,  how- 
ever, we  wish  to  find  how  much  convergence-effort  remains  alive, 
so  to  speak.  The  two  stimuli  capable  of  provoking  it  are  : 

(1)  Accommodation,  and 

(2)  The  mental  effort  to  converge  by  perceiving  a  near  object  : 
for  even  a  blind    person    can  converge,   by  mentally  conceiving  a 
»iear  object. 


Ocular  Paralyses  191 

Both  of  these  stimuli  to  convergence  (namely,  accommodation 
and  "knowledge  of  nearness  ")  are  introduced  together  when  we 
make  the  patient  regard  a  small  approaching  object. 

What  we  do,  therefore,  is  to  carefully  watch  the  squinting  eye 
to  see  if  the  squint  becomes  less  as  we  make  an  object  travel  straight 
toward  the  fixing  eye. 

If  desirable,  we  may  actually  measure  the  convergence-effort 
on  the  perimeter,  by  first  measuring  the  squint  with  distant  vision 
and  then  with  near  vision  of  two  or  three  small  letters  held  on  the 
end  of  a  projecting  arm  fixed  to  the  ivory  button  and  pointing 
toward  the  fixing  eye  and  sufficiently  near  to  it  to  excite  the  maxi- 
mum of  accommodation. 

It  is  but  rarely  that  we  need  do  this,  or  still  less  trouble  to  test 
the  proportion  between  the  accommodation  element  and  the  knowl- 
edge of  nearness  element  :  it  can,  however,  be  easily  done  by  placing 
a  concave  lens  before  the  fixing  eye  to  elicit  accommodation  while 
the  squint  is  measured  at  a  distance,  either  objectively  or  subjectively. 


CHAPTER   X 


Nystagmus 

Nystagmus  is  a  phenomenon  of  great  interest,  which,  if 
thoroughly  understood,  would  throw  much  light  on  the  nature  ot 
ocular  motions.  It  is  absolutely  involuntary,  almost  always  simul- 
taneous and  symmetrical  in  the  two  eyes  ;  and  even  when  it  is 
induced  by  making  one  eye  look  strongly  in  the  direction  of  a 
single  paralyzed  muscle,  so  as  to  obtain  the  slight  jerking  move- 
ment which  appears  when  the  limit  of  muscular  power  has  been 
reached  ("liminal  nystagmus"),  a  similar  movement  can  generally 
be  observed  in  the  sound  eye.  This  proves  that  the  weak  muscle 
is  not  the  seat  of  the  nystagmus,  but  only  its  occasion.  It  is  really 
the  conjugating  center  which  has  approached  the  limit  of  its  power 
in  the  effort  to  stimulate  an  irresponsive  muscle.  It  is  only  some 
kinds  of  nystagmus  that  can  correctly  be  described  by  the  word 
"oscillation."  In  the  great  majority  of  cases  the  movement 
resembles  that  of  the  flagellae  of  ciliated  epithelium,  the  movements 
being  quick  in  one  direction  and  slower  in  the  opposite.  In  hori- 
zontal or  vertical  nystagmus  the  quick  movement  is  generally  in 
the  direction  of  any  voluntary  movement  of  the  eyes  to  the  right 
or  left.  In  mild  cases  and  those  acquired  in  later  life,  the  nystag- 
mus appears  only  during  voluntary  movements  of  the  eyes  from 
the  primary  position,  and  this  is  especially  so  with  miners,  in  whom 
the  defect  makes  its  appearance  most  readily  when  the  eyes  are 
directed  upwards  into  the  position  required  for  their  work.  Nystag- 
mus ceases  during  sleep,  which  fact  is  very  instructive.  Just  as 
the  converging  innervation  surrenders  its  activity  during  sleep,  so 
as  to  make  ordinary  eyes  diverge,  and  convergent  squints  to  relax, 
so  the  other  parallel  innervations  give  up  their  tonic  activity  and 
nystagmus  ceases.  The  kinetic  energy  expended  throughout  the 
day  in  some  cases  of  nystagmus  is  really  very  great  and  shows  how 
considerable  are  the  activities  of  the  conjugate  centers. 

Varieties. — The  movements  in  nystagmus  are  so  very  various 
in  kind  as  to  point  strongly  to  the  separate  existence  of  the  several 
conjugating  centers  referred  to  in  a  previous  chapter.  ' '  Horizontal ' ' 
nystagmus  is  the  most  common.  Next  comes  "rotational"  nystag- 

(192) 


Nystagmus  193 

mus,  in  which  the  corneal  meridians  remain  parallel  with  each  other 
but  experience  simultaneous  dextrotorsion  and  laevotorsion.  Its 
existence  affords  evidence  of  two  conjugating  centers  for  torsion. 
"  Vertical"  nystagmus  is  less  frequent  still.  It  may  occur  alone, 
or  combined  with  lateral  movement  {mixed  nystagmus).  "Con- 
verging" nystagmus,  in  which  the  two  eyes  approach  each  other 
and  recede  again,  is  extremely  rare,  but  one  case  has  been  described 
by  Gowers  and  another  by  Sym.  Cases  of  "  unilateral"  nystagmus 
are  met  with  and  are  rather  difficult  to  explain,  but  they  often  are  not 
examples  of  true  jerking  nystagmus  but  only  of  trembling  of  the  eye. 

In  a  few  patients  one  kind  of  nystagmus  succeeds  another.  It 
may,  for  example,  be  horizontal  one  moment  and  rotary  the  next, 
and  would  be  well  called  " protean."  It  is  not  unlikely  that  in 
these  cases  one  kind  of  nystagmus  is  evoked  by  one  eye  and  the 
other  kind  by  the  other  eye,  and  that  each  takes  the  lead  alternately, 
imposing  its  own  character  of  nystagmus  on  the  other.  The  restless 
wanderings  of  totally  blind  eyes  in  adults  doubtfully  come  under  the 
heading  of  nystagmus-;  but  "searching"  nystagmus  (Sym)  is 
sometimes  seen  in  patients  of  this  kind,  "the  eyes  passing  from 
near  the  one  canthus  to  near  the  other  in  a  series  of  jerks,  to 
return  in  one  long  sweeping  movement." 

Rapidity. — The  rapidity  of  nystagmatic  motions  varies  from 
10  to  350  or  more  per  minute.  The  excursions  may  be  so  small  as 
to  be  only  visible  by  the  ophthalmoscope,  or  may  be  as  large  as 
one  centimeter.  Vertical  nystagmus  is  always  rapid  ;  so,  also,  is 
nystagmus  which  dates  from  early  childhood.  Nystagmus  due  to 
exhausted  innervations  in  adult  life  is  also  fairly  quick  ;  but  where 
it  is  simply  due  to  acquired  total  blindness,  it  is  more  frequently 
slow.  The  unilateral  nystagmus  of  highly-amblyopic  squinting 
eyes  is  usually  slow,  though  Javal  records  a  rapid  vertical  nystagmus 
in  an  eye  of  this  kind.  The  unilateral  nystagmus,  however,  which 
sometimes  accompanies  spasmus  nutans  is  rapid. 

Apparent  Motion. — Nystagmus  from  infancy  is  unaccompanied 
by  any  apparent  movement  of  objects,  though,  of  course,  their 
pictures  are  continually  moving  on  the  retina.  This  shows  that 
the  brain  is  conscious  of  the  movements,  estimates  their  measure 
and  allows  for  them.  It  is  probable  that  during  the  quick  snatches 
no  vision  is  effected,  and  only  during  the  slower  relapsing  move- 
ment does  it  take  place.  In  ''miners'  nystagmus"  objects  appear 
to  move  "either  in  a  circle  or  ellipse"  (Snell),  which  appears  to 


194  Tests  and  Studies  of  the  Ocular  Muscles 

show  that  only  the  lower  co-ordinating  centers  are  concerned. 
"  It  is  sometimes  accompanied  by  tremors  of  the  head  (perceptible 
to  the  hand  placed  thereon),  of  the  eyelids  and  of  the  muscles  of 
the  face  and  neck."  In  some  other  rare  forms  of  nystagmus  the 
head  moves  synchronously,  either  with  the  quick  motion  of  the 
nystagmus  or  against  it. 

Examination  Of  Nystagmus. — There  are  three  tests  which 
should  not  be  omitted. 

(1)  The  field  of  fixation,  which  may  be  found  limited  in  cer- 
tain directions,  corresponding  generally  to  the  positions  in  which 
the  nystagmus  is  worst. 

(2)  The  position  of  least  nystagmus  should  be  ascertained  for 
each  eye  separately,  by  presenting  a  small  object  for  the  patient  to 
scrutinize  (Javal). 

(3)  Each  eye  should  be  covered  in    turn,   to    notice  whether 
there  is  any  difference  in  the  character  and  time  of   the  motions 
under  the  regime  of  each. 

It  is  also  advisable  to  examine  the  macular  region  for  a  fine 
central  scotoma,  and  to  note  any  error  of  refraction.  In  cases  of 
total  color  blindness  nystagmus  is  almost  always  present,  since 
there  is  no  central  fixation,  the  macular  region  of  the  field  being 
represented  by  a  tiny  central  scotoma  (Nettleship)  due  to  inefficiency 
of  the  cones. 

Etiology. — The  chief  causes  may  be  classified  as  (a)  vision 
defects  ;  (£)  neuromotor  exhaustion  ;  {c}  organic  disease.  To 
this  might  be  added  a  few  rarer  causes,  such  as  reflexes,  ataxies,  etc. 

Visual  causes  account  for  the  greater  proportion  of  the  cases 
acquired  in  infancy.  It  is  essential  for  this  that  both  eyes  should 
have  their  vision  impaired  during  the  early  weeks  of  life  when, 
under  normal  conditions,  central  fixation  is  being  acquired.  The 
defects  of  vision  must  be  considerable  at  the  time,  though  they 
may  pass  away  afterwards,  leaving  the  nystagmus. 

Congenital  cataract,  ophthalmia  neonatorum,  or  central  chorido- 
retinitis,  are  the  commonest  causes.  In  adults,  either  complete 
blindness,  or  macular  disease,  is  necessary.  Even  ripe  cataract  is 
not  bad  enough  to  cause  it. 

Neuromotor  exhaustion  accounts  for  miners'  nystagmus,  which 
is,  therefore,  functional  rather  than  organic,  and  is  always  improved 
by  complete  change  to  an  occupation  no  longer  demanding  the 
same  direction  of  fixation.  It  is  analogous  to  writers'  cramp. 


Nystagmus  195 

Pathological  processes  which  cause  nystagmus  are  very 
numerous.  Almost  any  affection  of  the  brain  which  impairs  the 
vitality  of  the  conjugate  centers  may  cause  it,  and  especially  affec- 
tions of  the  pons,  cerebellum,  corpora  quadrigemina,  peduncles 
and  optic  thalami.  Affections  of  the  semicircular  canals  also 
cause  it  Sometimes  pressure  on  the  ear  will  cause  it,  and  in 
one  remarkable  case  of  ear  disease,  inflation  of  the  middle  ear 
invariably  caused  nystagmus  in  one  direction,  while  rarefaction  of 
the  air  caused  nystagmus  in  the  opposite  direction. 

Nystagmus  is  an  important  symptom  in  ' '  disseminated 
sclerosis,"  with  "intentional  tremor"  of  the  limbs  and  exagge- 
rated knee  jerks.  In  "paralysis  agitans,"  on  the  other  hand, 
where  the  movements  continue  during  rest  and  therein  differ  from 
those  of  nystagmus,  the  latter  does  not  occur.  The  presence  of 
nystagmus  (on  fixation  only)  in  Friederich's  ataxia  is  one  of  the 
distinguishing  marks  between  this  disease  and  locomotor  ataxia. 

Nature  Of  the  Excursions.— The  peculiar  character  of  the 
quick,  jerking  movement,  with  slower  return,  naturally  tempts 
inquiry  as  to  its  meaning.  It  is,  however,  though  unnoticed,  a 
constant  phenomenon  of  every-day  life.  As  we  walk  along  we  may 
look  at  the  ground  or  at  objects  around  us.  In  either  case,  or,  in 
fact,  whenever  there  is  relative  motion  between  our  bodies  and 
neighboring  objects,  the  eyes  fix  one  salient  point  after  another, 
experiencing  a  slow  motion  while  looking  at  each,  followed  by  a 
quick  motion  in  the  opposite  direction  in  order  to  gain  the  next. 
Thus,  when  we  look  on  the  ground  while  walking  along,  the  point 
of  fixation  does  not  run  evenly  along  the  ground,  like  a  little  dog, 
before  us,  but  is  quickly  transferred  from  point  to  point,  lingering 
on  each  long  enough  to  give  a  good  retinal  picture  but  not  too 
long  to  tire  the  retina.  It  is  reasonable  to  expect,  therefore,  that 
the  central  apparatus  should  be  so  constructed  as  to  facilitate  move- 
ments of  this  kind  and  that  when  the  centers  are  deranged  they 
should  occur  spontaneously. 

The  great  part  played  by  visual  defects  in  causing  nystagmus 
is  perhaps  to  be  explained  as  follows  :  All  ordinary  visual  actions 
combine  a  voluntary  element  with  an  involuntary  reflex  element. 
It  is  the  latter  which,  after  we  have  directed  our  eyes  to  an  object, 
retains  them  there  for  a  brief  period.  In  nystagmus  this  retain- 
ing power  is  lost,  so  that  the  gaze  is  no  sooner  brought  to 
bear  upon  an  object  by  the  usual  quick  movement  than  the  eye 


1 96  Tests  and  Studies  of  the  Ocular  Muscles 

moves  away  from  it  from  want  of  the  proper  physiological  sup- 
port of  reflex  fixation.  Any  defect,  therefore,  in  the  reflex  loop, 
whether  from  blindness  or  from  imperfect  development  or  educa- 
tion in  early  life,  or  from  central  or  motor  defects,  will  suffice  to 
cause  nystagmus. 

When  we  turn  round  and  round,  thesemicular  canals  inform  us 
of  the  motion,  and  the  eyes  at  the  same  time  are  caused  by  sur- 
rounding objects  to  make  movements  similar  to  those  of  nystagmus, 
the  slow  movements  being  opposite  to  that  of  the  body  and  the 
quick  ones  with  it.  We  can  easily  understand,  therefore,  that 
disease  of  the  semicircular  canals  may  cause  nystagmus,  since  the 
two  are  so  connected  in  daily  life.  Important  conjectures  have 
been  made  as  to  the  mechanisms  involved,  but  the  proof  is  not  yet 
complete. 

Treatment  Of  Nystagmus. — Operate  as  early  in  life  as  possible 
for  congenital  cataract  or  corneal  leucomato.  In  older  children 
correct  the  refraction,  to  give  more  rest  to  the  nerve  centers. 
Avoid  over-fatigue  of  the  eyes.  If  the  position  of  least  nystagmus 
for  each  eye  separately  be  such  that  operations  on  the  ocular 
muscles  might  make  them  coincide  and  other  muscular  weaknesses 
agree  therewith,  tenotomy  or  advancement  might  do  good  in 
exceptional  cases.  In  miners'  nystagmus,  or  that  brought  on  by 
over-use  of  the  eyes  in  certain  directions,  change  of  occupation 
with  tonic  treatment  of  the  system  generally,  are  indicated. 

Curiosities. — Gowers  records  a  case,  with  symptoms  of  cere- 
bellar  tumor  and  lateral  nystagmus,  in  which  the  pharynx  and 
larynx  were  the  seat  of  similar  movement;  "that  in  the  pharynx 
was  horizontal  towards  the  middle  line  :  in  the  larynx  there  was 
a  similar  lateral  movement  of  the  arytenoid  cartilages'."  The 
rate  of  movement  was  the  same  as  in  the  ocular  muscles,  180  per 
minute. 

Curiously  enough,  Javal  has  recorded  a  case  of  nystagmus  in 
which  the  oscillations  of  the  eye  were  arrested  by  the  instillation  of 
atropine  ;  while,  on  the  other  hand,  Zehender  records  an  instance 
in  which  the  instillation  of  eserin  provoked  nystagmus.  Peculiar 
varieties  of  nystagmus  are  met  with  in  association  with  the  ' '  head 
nodding"  of  infants,  which  occurs  as  a  transitory  affection  during 
the  first  year  of  life  after  the  second  month.  The  jerking  of  the 
head  may  be  lateral,  rotary  or  nutatory,  but  ceases  in  the  recumbent 
position  or  during  sleep,  herein  resembling  nystagmus  ;  as  also 


Nystagmus  197 

in  becoming  more  pronounced  when  the  attention  is  directed  to  an 
object.  The  nystagmus  is  frequently  unilateral,  and  is  "generally- 
more  marked  on  one  side  than  on  the  other.  Sometimes  there  is 
rotary  or  vertical  nystagmus  of  one  eye  and  distinctly  horizontal 
movements  of  the  other  "  (Thomson).  Holding  the  head  steady 
generally  increases  the  nystagmus.  In  some  cases  there  is 
rythrnical  contraction  and  dilation  of  the  pupils.  The  occasional 
spells  of  lateral  deviation  of  the  head  and  eyes,  sometimes  associated 
with  unconsciousness,  suggestively  indicate  the  mechanisms  affected. 


CHAPTER   XI 


Ophthalmoscopic  Corneal  Images 

Though  the  corneal  reflections  of  candles,  tapers,  etc.,  have 
been  used  for  a  very  long  time,  I  believe  the  first  published  use  of 
a  reflection  from  the  ophthalmoscope  was  by  Priestley  Smith,  for 
his  well-known  tape  method  of  strabismometry,  about  fifteen 
years  ago. 

When  an  eye  is  illuminated  by  reflection  from  an  ophthalmo- 
scope, a  brilliant  spot  of  light  appears  to  rest  on  the  cornea,  pro- 
duced, in  reality,  by  reflection  from  the  film  of  liquid  on  its  surface, 
as  from  a  strong  convex  mirror.  This  corneal  reflection  is  shown 
in  Fig.  78.  The  beginner  in  ophthalmoscopy  is  only  too  well 
acquainted  with  this  spot,  which  gets  in  his  way  whenever  he 
attempts  to  explore  the  fundus.  It  is  well,  therefore,  to  know  its 
powers  for  good  as  well  as  evil,  and  in  how  great  a  variety  of  ways 
the  very  same  reflection  can  be  turned  to  account. 

Corneal  images  produced  by  light  reflected  from  a  perforated 
mirror,  such  as  that  of  an  ophthalmoscope,  afford  far  greater  pre- 
cision in  the  clinical  investigation  of  the  position  of  an  eye  than 
corneal  images  formed  in  any  other  way,  provided  attention  be 
given  to  one  or  two  simple  details. 

First  Precaution. — The  first  precaution  I  like  to  insist  on  is 
that  the  patient's  attention  should  be  directed  to  the  mirror,  and 
preferably  to  its  central  aperture.  Under  these  conditions  the 
visual  line  of  an  observed  eye  coincides  with  the  visual  line  of  the 
observer's  eye  and  the  spot  of  light  maps  out  with  sufficient  pre- 
cision for  clinical  purposes,  the  point  in  each  cornea  which  is  tra- 
versed by  the  visual  line. 

Since  the  visual  line  does  not  generally  traverse  the  center  of 
the  cornea,  it  is  extremely  convenient  to  have  a  way  of  so  simply 
seeing  where  it  lies  in  different  eyes. 

The  Second  Precaution  is  to  avoid  being  misled  by  anomalies 
in  the  pupils,  which  are  often  placed  differently  in  the  two  eyes. 
It  is  the  position  of  the  corneal  reflection  relative  to  the  cornea, 
and  not  relative  to  the  pupil,  which  we  require  to  know,  though 
we  may  conveniently  use  the  pupil  as  a  landmark  by  first  noting 

(193, 


Ophthalmoscope c  Cornea!  Images  199 

the  distance  of  its  inner  and  outer  margins  from  the  corneal 
margin  and  then  the  place  occupied  within  the  pupil  by  the 
corneal  reflection.  It  is  generally,  however,  better  to  ignore  the 
pupil  altogether. 

One  great  advantage  of  the  ophthalmoscope  for  these  reflec- 
tions is  that  it  is  equally  available  in  daylight  and  artificial  light. 
In  daylight  the  patient  should  stand  with  his  back  to  the  window 
and  preferably  at  a  good  distance  from  it,  while  the  observer, 
facing  the  window,  reflects  the  light  from  it  by  the  ophthalmo- 
scope on  to  first  one  eye  and  then  the  other,  directing  the  patient's 
attention  to  the  aperture  in  the  mirror  that  he  (the  observer)  looks 
through.  With  artificial  light,  as  when,  for  instance,  the  patient 
is  seated  for  ophchalmoscopic  exploration  of  the  fundus,  the  best 
distance  is  from  eight  to  twelve  inches. 

If  we  wish  to  decide  whether  a  patient's  squint  is  "  apparent" 
or  real,  we  have  only  to  flash  the  light  on  to  first  one  eye  and  then 
the  other.  If  the  corneal  images  occupy  symmetrical  positions  in 
the  two  corneae,  as  in  Fig.  78,  no  squint  exists,  and  in  such  a  case 


Normal  eye.     (Photographed  by  author's  squint  camera.)     To  show  the  Corneal  Image  in  an 

average  eye  ;  the  pupil  displaced  slightly  inwards  and  the  corueal  image  displaced 

still  more  inwards. 

the  cause  of  an  apparent  squint  will  at  once  be  evident  from  the 
fact  that  the  corneal  images  in  both  eyes  will  be  found  to  be  sym- 
metrically displaced  from  the  usual  fixation  position,  either  both 
too  much  inwards  or  both  too  much  outwards.  But  while  symmet- 
rical displacement  explains  apparent  squint,  marked  unsymmetrical 
displacement  shows  real  squint  to  exist. 

\Yith  babies  the  test  is  of  special  service,  for  though  they  can- 
not, of  course,  fix  the  central  aperture,  they  are  fascinated  by  the 
bright  light  from  the  mirror,  which  answers  almost  as  well,  and  then 
by  rapidly  flashing  the  light  from  one  eye  to  the  other,  it  is  easy  to 
see  not  only  whether  any  deviation  exists  but  also  which  is  the 

*  Figs.  7">.  Tfi  and  77  in  the  original  work  were  omitted  by  the  author  in  this  revised  edition, 
which  explains  the  break  iu  the  sequence  of  the  figures  at  this  point. 


200  Tests  and  Studies  of  the  Ocular  Muscles 

squinting  eye.  To  be  expert,  a  little  practice  is  necessary,  but  the 
same  is  true  of  every  method  of  examining  the  eyes.  Many  a 
perplexity  would  be  at  once  dispelled  if  these  corneal  images  came 
to  the  rescue. 

Fixation  Position  of  the  Corneal  Reflection. — When  the  vision  of 
babies  is  imperfect,  or  the  two  eyes  do  not  work  well  together,  it  is 
easy  to  find  whether  each  eye  possesses  the  power  of  central  fixation 
by  observing  whether  each  corneal  image  occupies  the  ' '  fixation  posi- 
tion," with  steadiness.  In  order  to  describe  the  "  fixation  position," 
let  us  mention  a  third  precaution  to  be  observed — namely,  to  allow 
for  the  imperfect  collimation  of  the  visual  line*  and  its  variations. 
We  will,  for  simplicity,  suppose  that  the  eye  has  only  two  axes,  as 
in  Fig.  79,  viz. ,  ( i )  the  geometrical  axis, 
and  (2)  the  axis  of  vision,  and  explain 
these  briefly  : — 

The  optic  axis  (  G)  is  the  geometrical 
axis  on  which,  so  to  speak,  the  eye  is 
built,  passing  from  the  center  of  the  cornea 
in  front  to  the  posterior  pole  of  the  eye 
behind.  With  this  axis,  however,  the  line 
of  vision  (  V)  does  not  coincide,  for, 
curiously  enough,  we  do  not  see  straight 
out  of  our  eyes,  but  obliquely  out  of  them. 
This  is  due  to  the  fact  that  the  "  fovea 

To  show  the  obliquity  of  the  ,. 

visual  Axis  (v f)  with       centralis      (  /  )  does  not  lie  exactly  at  the 

reference  to  the  Geometri-  .  i      •  •     i  N  ,- 

cai  Axis  («/>).  The  Fovea       posterior  pole  of  the  eye  (  f>).  but  slightly 

(/)  is  to  the  outer  side  of  .  ,   ,     ,          • 

the  posterior  pole  ( p).  to  its  outer  side  and  below  it. 

Consequently,  the  line  of  vision  (  V) 

intersects  the  geometrical  axis  at  the  nodal  points  (<?)  of  the  eye, 
as  shown  at  Fig.  79  (where,  for  simplicity,  the  two  nodal  points 
are  reduced  to  one),  and  then  traverses  the  cornea  to  the  inner 
side  of  its  center.  In  consequence  of  this  the  corneal  image, 
visible  while  the  patient  looks  at  the  center  of  the  ophthalmoscopic 
mirror  (and  which,  as  we  have  already  said,  approximately  maps 
outjor  us  on  the  cornea  the  point  traversed  by  the  visual  axis), 

™i  *Thi*»'S  ^  exactly  the  angle  gamma,  though  it  may  he  assumed  to  be  practically  identi- 

he  angle  alpha  is  the  angle  between  the  visual  line  and  the  major  axis  of  the 

llipsoid  (now  disproved),  while  the  angle  gamma,  is  the  angle  between  the  line  of 

>  the  optic  axis.     The  visual  line  passes  from  the  point  looked  at  through  the  nodal 

e  fovea.    The  line  affixation,  on  the  other  hand,  extends  from  the  point  looked  at 

and    h»   ,,?i      motlon.of  the  eye.    The  discrepancy  between  the  aberration  of  the  visual  line 

Doniil •  V  ,,,,'       ^amrna  in  a  given  eye  is  greater  in  proportion  to  the  nearness  of  the  object, 

g/f  alpha,  since  be  assumed  the  major  axis  of  the  corneal  ellipsoid  to  coincide  with 

uU-aft  axis  of  the  eyeball,  is  the  angle  between  this  axis  and  the  visual  line. 


Ophthalmoscopic  Corneal  Images  201 

appears  to  the  inner  side  of  the  center  of  each  cornea  (see  Fig.  78). 
The  average  aberration  of  the  visual  line  is,  in  emmetropia,  5°. 
In  hypermetropia  the  angle  is  greater,  the  average  given  by 
Donders  being  nearly  8°,  and  in  myopia  it  is  less,  sometimes 
even  negative,  the  average  given  by  Donders  being  less  than  2°. 
From  the  fact  that  Donders  called  this  angle  the  angle  alpha,  some 
confusion  has  arisen  in  the  use  of  that  term.  (See  foot  note,  p.  200.  ) 

Apparent  Squint. — In  consequence  of  these  differences,  hyper- 
metropic  eyes  appear  slightly  divergent  and  myopic  eyes  slightly 
convergent,  for  we  are  so  accustomed  to  the  emmetropic  aberration 
as  to  think  any  greater  or  less  aberration  peculiar,  and  hence  arise 
the  two  well-known  varieties  of  "apparent  squint."  The  apparent 
position  of  the  corneal  image  on  the  cornea,  while  the  center  of  the 
mirror  is  fixed  by  the  patient,  may,  as  already  mentioned,  with 
advantage  be  called  the  "fixation  position"  of  the  image. 

We  have  seen  that  in  emmetropia  the  fixation  position  is  to 
the  inner  side  of  the  corneal  center  ;  in  hypermetropia  it  is  still 
farther  to  the  inner  side,  because  the  angle  gamma  is  greater  ;  in 
myopia  it  is  less  to  the  inner  side  or  even,  in  some  cases,  slightly 
to  the  outer  side  of  the  corneal  center,  because  the  angle  gamma  is 
smaller  or  even  negative.  In  emmetropia  the  most  common  condi- 
tion is,  as  represented  in  Figs.  78  and  80,  for  the  pupil  to  be  slightly 
to  the  inner  side  of  the  center  of  the  cornea,  and  for  the  corneal 
image  to  be  again  slightly  to  the  inner  side  of  the  center  of  the 
pupil.  It  is  important,  however,  not  to  trust  much  to  the  position 
of  the  pupil  lest  it  should  mislead,  and  if  the  pupil  be  misplaced 
the  position  of  the  image  in  the  cornea  should  be  studied  rather 
than  its  position  in  the  pupil.  In  an  eye  free  from  nystagmus  and 
which  possesses  the  power  of  central  fixation,  the  corneal  image 
occupies  the  fixation  position  with  great  steadiness.  If  central 
fixation,  however,  be  lost,  the  image  is  seen  to  wander  aimlessly 
about  the  cornea,  though  really,  of  course,  it  is  the  cornea  itself 
which  wanders. 

Priestley  Smith  has  made  the  interesting  and  valuable  observa- 
tion that  in  tobacco  amblyopia  the  power  of  central  fixation  is 
retained,  while  in  some  cases  of  acute  retro-bulbar  neuritis  it  is 
lost.  An  absolute  scotoma  involving  the  macula  would,  of  course, 
destroy  central  fixation,  which  also  might  very  likely  be  impaired  by 
functional  or  organic  changes  at  the  macula,  produced  by  looking 
at  strong  light  or  by  over-use  of  the  microscope,  etc. 


202 


Tests  and  Studies  of  the  Ocular  Muscles 


Refraction  Surmisable.— With  a  little  practice  it  is  quite  easy 
to  surmise  from  the  corneal  image  alone  whether  an  eye  is  much 
hypermetropic  or  myopic,  and  I  have  pointed  out  elsewhere  that  a 
high  angle  gamma,  as  indicated  by  an  unusually-displaced  corneal 
image,  should,  in  an  apparently  emmetropic  eye,  make  us  suspect 
the  presence  of  latent  hypermetropia  and  induce  us  to  paraly?e  the 
accommodation.*  It  is  well,  however,  to  remember  that  exceptions 
to  the  rule  are  not  infrequent. 

The  angle  gamma  in  astigmatism  does  not  appear  to  have  been 
studied  fully  yet.  In  some  cases  of  hypermetropic  astigmatism  in 
which  the  deficient  curvature  was  horizontal,  I  noticed  a  greater 
angle  alpha  than  in  emmetropia  ;  and  my  impression  is  that,  as  a 
rule,  a  cornea  which  is  too  flat  horizontally  has  a  higher  angle 
gamma  than  usual,  whatever  the  vertical  meridian  may  be. 

The  beauty  of  ophthalmoscopic  corneal  images  is  that  we  are 
able,  as  it  were,  to  actually  see  in  a  moment  what  point  of  the  cornea 
is  traversed  by  the  line  of  vision  (cf.  Figs.  78  and  79),  and  by  the 
distance  at  which  this  point  lies  from  the  center  of  the  cornea  to  guess 
approximately  the  amount  of  the  angle  gamma.  Any  instance  of  an 
unusually  high  or  low  angle  at  once  strikes  us  and  should  set  us  to 
try  and  account  for  it  by  looking  for  some  abnormal  condition  of 
refraction,  eccentric  fixation  or  unusual  shape  of  the  eye. 

Clinical  acknowledgment  of  the  gamma  is,  I  believe,  the  key  to 
the  successful  use  of  ophthalmoscopic  corneal  images,  and  it  is  this 
which  enforces  the  necessity  of  the  patient's  attention  being  directed 
to  the  mirror  and,  if  possible,  to  its  central  aperture,  since  then,  in 
normal  eyes,  the  two  images  are  symmetrical  (Fig.  80).  If  the 
same  eyes  be  allowed  to  wander  to  one  or  the  other  side,  the 
images  will,  of  course,  appear  unsymmetrical,  for  one  will  be 
nearer  the  edge  of  its  cornea  than  the  other,  by  a  distance  equal 
to  twice  the  monocular  aberration  (Fig.  81).  The  vertical  element 
of  the  angle  alpha,  shown  by  the  corneal  image  lying  generally 
slightly  above  the  horizontal  diameter  of  the  cornea,  seems  of 
less  clinical  importance,  and  it  is  often  imperceptible,  though 


Ophthalmoscopic  Cornea!  Images  203 

its  amount  is  also  subject  to  variation  ;  I  have  not  devoted  much 
attention  to  it,  though  noting  many  cases  of  very  marked  vertical 
displacement. 

Angle  Gamma  in  Cataract  and  Iridectomy.— It  is  very  pretty 
to  see  how  faithfully  the  corneal  image  occupies  its  correct  "fixa- 
tion position  "  in  cases  of  lamellar  cataract  not  quite  large  enough 


.Fig.  SO 


Fig.  81 


To  show  the  Symmetry  of  the  images  when  normal  eyes  look  straight  at  the  mirror  and  the 
Asymmetry  of  the  images  when  the  same  eyes  look" away  from  the  mirror,  though  the  eyes 
are  iiot  squinting.  Fig.  80  shows  how  and  Fig.  81  how  not  to  use  corneal  images. 

to  fill  the  pupil,  even  though  the  reflection .  lies  against  the  most 
opaque  portion  of  the  cataract.  The  visual  line,  therefore,  tra- 
verses the  cataract,  as,  of  course,  it  would  on  simple  optical 
principles.  Similarly,  in  cases  of  very  peripheral  iridectomy  for 
occluded  pupil,  and  when  the  iris  is  drawn  to  one  side,  as  in  old 
cases  of  prolapse,  the  corneal  image  still  occupies  its  proper  posi- 
tion, though  against  an  opaque  background,  and  demonstrates, 
perhaps  more  prettily  than  anything  else  could  do,  the  fallacy  of 
supposing  that  a  nasal  or  temporal  iridectomy  predisposes  to  stra- 
bismus or  alters  materially  the  relations  between  convergence  and 
accommodation. 

Unsymmetrical  Angles  Gamma. — Now  let  us  consider  a  dif- 
ficulty in  the  detection  of  strabismus  by  corneal  images  which  arise 
very  occasionally.  The  angle  gamma  may  be  different  in  the  two 
eyes,  so  that  the  corneal  images  appear  unsymmetrical.  The 
asymmetry  in  these  cases  is,  however,  so  slight  that  its  very 
smallness  leads  us  to  suspect  its  true  cause,  and  if  we  place  the 
hand  over  each  eye  in  turn,  it  will  be  found  that  the  "  fixation 
position  "  is  not  the  same  in  each.  Why,  it  may  be  asked,  does 
the  very  smallness  of  the  asymmetry  lead  us  to  suspect  its  true 


204  Tests  and  Sttidies  of  the  Ocular  Muscles 

cause  ?  The  answer  is  :  Because  minute  squints  are  exceed- 
ingly rare,  except  when  one  eye  is  blind  or  its  image  ignored, 
the  natural  desire  for  single  vision  being  too  strong  to  allow 
minute  squints  to  exist  without  considerable  efforts  being  made  to 
overcome  them. 

In  cases  of  alleged  recent  monocular  blindness,  the  presence 
of  a  very  slight  squint  affords  presumptive  evidence  of  the  veracity 
of  the  patient,  since  a  slight  persistent  squint  cannot  be  voluntarily 
created.  As,  for  instance,  in  the  case  of  a  young  woman  who 
stated  that  till  a  few  days  before  she  presented  herself  she  had 
perfect  sight  in  both  eyes  and  that  suddenly  the  sight  of  the  left 
eye  disappeared.  No  change  could  be  detected  in  the  fundus  and 
the  pupil  reacted  normally,  so  that  the  case  looked  like  one  of 
feigned  amblyopia.  Ophthalmoscopic  corneal  images,  however, 
showed  that  there  was  a  minute  squint,  and  this  corroborated  the 
patient's  statement. 

Alternation. — A  "  monolateral' '  squint  is  one  in  which  the 
same  eye  always  fixes  and  the  other  always  squints,  in  contrast  to 
an  "alternating  "  squint,  in  which  latter  either  eye  fixes  indifferently. 
In  squints  of  high  degree  it  is  most  easy  to  determine  whether  they 
are  alternating  or  monolateral,  without  the  aid  of  corneal  reflections, 
by  simply  covering  the  fixing  eye  for  a  few  moments,  so  as  to  make 
the  other  one  take  up  fixation  instead  ;  if  the  latter  continues  to  fix 


Fig.  82 

Rather  small  Angle  Gamma,  especially  in  left  eye,  in  a  case  of  low  myopia  (.5  D.). 

when  uncovered,  the  squint  is  alternating,  but  if  fixation  is  at  once 
transferred  back  to  the  originally  fixing  eye,  the  squint  is  mono-- 
lateral. With  minute  squints,  however,  it  is  not  so  easy  to  settle 
this  point  without  the  aid  of  corneal  images,  which  enables  us  at 
once  to  see  which  is  the  fixing  eye  and  whether,  by  covering  this 
eye  temporarily,  fixation  can  be  transferred  to  the  other. 

Concomitancy. — A  still  more  important  point  to  settle  is  that  of 
"  concomitancy,"  because  by  this  alone  can  we  tell  whether  or  not 


Ophthalmoscopic  Cornea/  Images  205 

a  squint  is  paralytic.  In  paralytic  squint  the  degree  of  strabismus 
increases  on  looking  in  the  direction  of  action  of  paralyzed  muscle  ; 
whereas,  in  concomitant  squint,  the  degree  remains  the  same  in 
whatever  direction  the  patient  looks.  The  following  method  is  one 
which  I  have  found  useful  :  Lay  the  palm  of  the  left  hand  on  the 
patient's  head,  with  instructions  to  let  the  head  follow  the  most 
gentle  guidance  of  the  hand  without  resistance.  Now  note  the 
exact  position  of  the  corneal  reflex  in  the  squinting  eye  while  the 
fixing  eye  is  directed  to  the  central  aperture  of  the  mirror,  and 
steadily  turn  the  head  to  the  right  and  left,  up  and  down  and  into 
intermediate  positions,  to  notice  whether  the  position  of  the  reflec- 
tion is  unchanged  by  these  manoeuvres.  If  it  is  unchanged,  the 
squint  is  concomitant ;  if  otherwise,  the  squint  is  paralytic,  pro- 
vided that  the  movements  made  are  not  too  great  to  bring  in  the 
fallacy  of  mechanical  impediment  from  one  of  the  corneae  reaching 
to  its  motor  limits.  Vertical  squints  are  just  as  easily  detected  as 
horizontal  ones. 

Test  for  Binocular  Fixation. — The  next  use  of  corneal  images 
to  describe  is  one  which  I  have  sometimes  found  of  value,  viz. ,  to 
test  for  binocular  fixation  when  its  existence  is  doubtful. 

After  operating  for  strabismus  and  setting  a  squinting  eye 
apparently  perfectly  straight,  we  are  often  at  a  loss  to  be  sure 
whether  both  eyes  are  able  to  work  together.  We  have  some 
interest  in  finding  this  out,  because  binocular  vision  is  so  great  a 
preservative  from  any  return  of  the  strabismus,  and  we  can  give  a 
better  prognosis  accordingly.  By  subjective  tests  it  is  often  impos- 
sible to  settle  the  question,  the  patients  being  so  frequently  either 
too  young  or  unintelligent  to  give  us  any  assistance.  An  objective 
test,  even  though  difficult  and  requiring  a  rather  detailed  descrip- 
tion, is,  therefore,  a  great  help. 

After  operation,  for  some  weeks  at  least,  the  eye  operated  on 
remains  more  stationary  than  its  fellow  (Berry),  so  that  by  turning 
the  head  slowly  to  the  right  or  left  we  make,  if  binocular  vision  is 
absent,  the  corneal  image  on  the  squinting  (and  operated)  eye 
slowly  and  steadily  move  across  part  of  the  cornea.  If  binocular 
vision  be  present,  it  may  be  strong  enough  to  overcome  the 
sluggishness  of  the  squinting  eye,  in  which  case  its  image  remains 
in  the  ' '  fixation  position  ' '  throughout.  But  even  if  the  desire  for 
single  vision  is  not  strong  enough  to  effect  this,  there  is  always,  if 
it  be  present  at  all,  a  part  of  the  field  of  fixation  over  which  the 


206 


Tests  and  Studies  of  the  Ocular  Muscles 


"fixation  position"  is  maintained,  and  at  the  edge  of  this  region 
the  corneal  image  suddenly  moves  to  another  point.  It  is  the  con- 
tinued maintenance  of  the  fixation  position  during  lateral  movements 
of  the  head  or  else  the  sudden  abandonment  of  the  fixation  posi- 
tion, instead  of  only  gradually  moving  away  from  it,  on  which  to 
count  in  making  the  test* 

To  Roughly  Measure  a  Squint.— Hirschberg  has  shown  that 
when  the  corneal  reflection  of  a  flame  occupies  the  margin  of  a 
medium-sized  pupil  (3^  mm.)  the  amount  of  squint  present  is  15° 
to  20°,  and  if  it  occupies  the  margin  of  the  cornea  about  45°.  This 
convenient  mode  of  guessing  the  amount  of  squint,  of  course, 
neglects  the  aberration  of  the  visual  line,  for  with  normal  aberration 
the  corneal  reflection  lies  nearer  the  inner  than  the  outer  margin 
of  the  cornea,  so  that  a  pupillary  marginal  reflection  means  a 
smaller  divergent  squint  and  a  greater  convergent  one  than  the 
mean  calculation.  It  is  easy,  however,  to  notice  what  the  aber- 
ration actually  is  and  to  allow  for  it. 

Priestley  Smith's  Mode  of 
Strabismometry. — This  excellent 
procedure  was  published  so  early 
as  1888.  A  piece  of  tape  i  m. 
(or  60  cm. )  long,  of  which  one 
end  is  held  by  the  patient  against 
his  temple,  while  the  other  end 
is  attached  to  a  ring  on  the  sur- 
geon's finger,  maintains  the  re- 
quisite distance  between  surgeon 
and  patient.  A  second  piece  of 
tape,  graduated  and  figured,  is 
attached  by  one  end  to  the  same 
ring  and  then  passed  between 
the  fingers  of  the  surgeon's  free 
hand,  at  which  the  patient  is 
directed  to  look.  When  the 
separation  of  the  surgeon's 
hands  reaches  the  measure  of 
the  squint  the  corneal  reflection 
occupies  the  normal  position  of  the  cornea  of  the  squinting  eye. 

Different   Points   Of    View. — Students    and   onlookers   some- 


Fig.  83 

Priestley  Smith's  Tape  Method.  The  rights 
hand  figure  shows  a  diverging  squint  and 
the  left-hand  figure  a  converging  one, 
both  of  the  right  eye.  The  ophthalmo- 
scope is  at  O  and  the  surgeon's  hand  at 
//.  When  the  fixing  eye  (L)  is  made  to 
look  at  the  surgeon  s  hand  the  squint- 
ing eye  (R)  becomes  straight  fur  the 
ophthalmoscope. 


*  "Ed.  Med,  Journ.,"  loc.  eii. 


Ophthalmoscopic  Cornea/  Images  207 

times  forget  that  they  do  not  see  the  corneal  reflections  under  the 
same  conditions  as  the  eye  behind  the  ophthalmoscope.  Fig.  84 
shows  a  convex  mirror  illuminated  from  a  point  L.  An  eye 
placed  at  7  sees  an  image  at  i,  an  eye  placed  at  //  an  image  at  2, 
at  ///  at  3,  at  IV  at  4,  at  V  at  5,  and  so  on  ;  the  reflections  lying 
in  a  caustic  curve. 

Error  Of  Approximation.— So  far,  I  have  assumed  that  the 
spot  of  light  on  the  cornea  marks  the  very  point  of  its  transit  by 
the  visual  line.  The  assumption  involves  an  exceedingly  small  error. 


Fig.  84 
The  Caustic  Curve  of  a  convex  mirror  whose  principal  focus  is  at  F 

That  the  approximation  should  be  trusted  requires  an  analysis  of 
the  exact  amount  of  error  and  its  nature.  Fig.  85  shows  the 
principles  involved. 

A  straight  line  connecting  the  center  of  curvature  (r)  of  the 
cornea  with  the  center  of  the  sight-hole  in  the  ophthalmoscope  is 
the  line  which  passes  through  the  apparent  center  of  the  corneal 
reflection.  Another  straight  line  connecting  the  anterior  nodal 
point  (A7")  with  the  center  of  the  sight-hole  is  the  visual  line.  It 
will  be  seen  that  they  do  not  quite  coincide,  and  traverse  the 
cornea  at  slightly  different  points,  the  difference  being  exaggerated 
in  the  figure  to  make  it  evident. 

Without  troubling  the  reader  with  calculations  on  so  insignifi- 
cant a  subject,  I  make  the  corneal  transit  of  the  visual  line  at  a 


208 


Tests  and  Sttidics  of  the  Ocular  Muscles 


Fig.  85 

To  show  that  the  Axis  of  Reflection  (r  c)  is  a  little 
farther  from  the  anterior  pole  than  the  Visual 
Axis  (v  A'),  c  is  the  center  of  curvature  of  the 
cornea  and  TV  the  uodal  point. 


distance  from  the  anterior  pole  of  the  eye,  which  is  only  seven- 
eighths  of  that  of  the  center  of  the  corneal  reflection.  Hence,  in 
emmetropia,  where  the  corneal  transit  of  the  reflection  is  displaced 

on  an  average  .63  mm. 
inwards,  the  displacement 
of  the  visual  line  is  seven- 
eighths  of  this,  the  error  of 
approximation  being  only 
TfT-  of  a  millimeter. 

The  corneal  reflection, 
therefore,  very  slightly  ex- 
aggerates the  real  devia- 
tion of  the  visual  line,  but 
since  it  does  so  in  a  uni- 
form proportion  of  8  to  7, 
there  is,  for  clinical  pur- 
poses, no  disadvantage  in 
it.  Since  the  fixation  line  proceeds  from  the  center  of  motion 
to  the  object,  the  corneal  reflection  lies  somewhere  between  the 
fixation  and  the  visual  lines. 

Photography  Of  Muscular  Anomalies. — Hitherto,  for  permanent 
records  of  ocular  paralyses,  oculists  have  had  to  confine  themselves 
pretty  much  to  subjective  hand-made  charts  of  diplopia. 

It  is  evident  that  photographic  charts  of  the  objective  position 
of  the  eyes,  free  from  all  the  fallacies  of  a  subjective  investigation, 
would  be  much  better  in  some  cases,  and  if  carefully  and  properly 
utilized,  the  corneal  reflections  afford  beautifully  precise  indices  of 
ocular  deviations  of  every  kind  except  torsional. 

Gullstrand  (1892)  made  a  number  of  photographs  of  muscular 
defects,  utilizing  the  reflection  from  an  ordinary  window  ;  but  this 
source  of  illumination  is  not  precise  enough  to  afford  such  good 
results  as  are  shown,  for  example,  in  Fig.  86,  by  marking  out  a 
much  smaller  point  on  each  cornea  through  which  the  fixation  line 
passes. 

We  have  seen  that  to  get  the  best  results  from  ophthalmoscopic 
corneal  reflections,  it  is  essential  that  the  patient  should  direct  his 
attention  to  the  central  aperture  of  the  mirror  (Figs.  80,  81)  and 
the  surgeon's  eye  should  be  behind  the  virtual  source  of  light 
(Fig.  84).  To  photograph  the  reflections  perfectly,  therefore,  the 
light  should  proceed  from  the  center  of  the  photographic  lens  or 


Ophihalmoscopic  Corneal  Images  209 

else  from  an  area  surrounding  the  lens  symmetrically,  though  indeed 
there  would  not  be  much  error  in  lighting  a  flame  or  incandescent 
lamp  just  over  the  lens,  while  making  the  patient  look  at  a  point 
midway  between  the  two. 

Since  daylight  is  the  best,  the  camera  which  I  have  designed 
for  the  purpose  (shown    in  Fig.   87)  will   probably  be  found   the 


Fig.  86 

High  Angle  Gamma  in  left  eye,  with  ascending  convergent  Sqirut  iu  the  right. 

handiest  kind  of  apparatus  to  use,  since  it  is  meant  for  work  out  of 
doors  where  the  greater  intensity  of  light  shortens  the  exposure — a 
point  of  great  importance  with  so  restless  an  organ  as  the  eye. 
An  elliptical  mirror  (m  n)  provided  with  an  elliptical  perforation 
nearer  its  lower  than  its  upper  end  is  fastened  at  an  angle  of  45° 
to  a  short  cylinder  of  wood.  This  short  cylinder  is  perforated  and 
provided  with  a  rapid 
portrait  lens  and  pneu- 
matic shutter.  The 
wooden  cylinder  can  be 
revolved  round  its  axis, 
so  as  to  bring  the 
brightest  part  of  the  sky 
into  view.  To  the  patient 

.  .  Author's  Squint  Camera. 

the  mirror,  owing  to  its 

inclination,  appears  perfectly  circular  and  makes  a  circular  reflec- 
tion on  the  cornea  with  a  small  black  dot  in  the  middle.  Since 
the  upper  part  of  the  mirror  is  farther  from  the  patient  than  the 
lower,  equal  lengths  there  subtend  smaller  angles.  For  this  reason 
the  perforation  of  the  mirror  should  be  nearer  its  lower  than  its 
upper  end,  according  to  a  simple  calculation.  The  plane  of  the  mirror 
should  pass  through  the  center  of  the  photographic  lens,  and  the 
major  axis  of  the  ellipse  should  be  its  minor  as  about  10  to  7. 

If  d  be  the  distance  of  the  patient  from  the  lens  and  b  be  the 
breadth  of  the  mirror,  let  m  represent  the  required  length  of  the 


2io  Tests  and  Studies  of  the  Ocular  Muscles 

mirror  above  the  center  of  the  lens  and  n  its  length  below,  6  the 
inclination  of  the  mirror  to  the  horizontal  and  a  the  angle  it  sub- 
tends at  the  eye.  Then 

b  cos.  \  a 

~  2  sin.  (  e~^-~y«) 

b  cos.  \  a 


2  sin.  (8  +  £  a) 
Therefore, 

m  Sin.  (0  -f  \  a) 


n  Sin.  (0  —  \  a) 

and,  since 

b  —  2  rf  tan.  |  a 

a  =    2   tan.  —1  — 
2  d 

From  these  formulae  it  is  quite  easy  to  construct  a  mirror  for 
any  inclination  that  may  be  most  convenient,  to  subtend  any 
given  angle  at  the  eye,  and  to  appear  as  a  perfect  circle  to  it. 

In  practice,  the  use  of  my  camera  has  given  me  much  pleasure, 
for  it  only  takes  a  minute  or  two  to  use.  To  save  time,  it  is  made 
in  the  form  of  a  wooden  box  with  a  fixed  focus.  The  patient's 
distance  is  adjusted  by  a  stick  of  the  right  length  and  the  box  is 
provided  with  an  Eastman  film  roll  holder.  To  use  the  camera  in 
the  consulting  room,  seat  the  patient  at  one  corner  of  the  window 
with  the  side  of  his  face  about  a  foot  from  the  glass.  Place  the 
camera  in  front  of  him  (also  about  a  foot  from  the  glass)  and 
with  the  mirror  rotate  45°  to  catch  the  sky  light  and  reflect  it  into 
his  eyes.  Bid  the  patient  look  at  the  center  of  the  lens,  adjust  the 
distance  of  the  camera  with  the  stick  and  give  about  three-seconds' 


Fig.  88 

Slight  (temporary)  over-correction  of  congenital  defect  of  Right  Superior  Rectus,  by  advance- 
ment of  the  Superior  and  tenotomy  of  the  Interior  Rectus. 

exposure.  Lately  I  have  used  the  stand  of  Javal's  ophthalmometer 
for  the  purpose.  The  instrument  is  removed  and  its  place  taken 
by  a  wooden  platform  for  the  camera  to  rest  on.  The  patient 
rests  his  chin  and  forehead  as  usual.  A  tiny  circle  of  paper  affixed 


Ophthahnoscopic  Corneal  Images  2ii 

to  the  center  of  the  lens  for  the  patient  to  look  at,  does  not  impair 
the  definition  of  the  photograph. 

For  those  who  have  not  a  special  camera,  it  may  be  well  to 
know  that  a  mere  disk  of  cardboard  encircling  the  lens  of  any 
ordinary  camera  suffices  to  give  a  reflection  from  the  cornea  out  of 


Fig.  89 

Chart  of  the  Corneal  Reflections  of  the  right  eye,  in  a  case  of  congenital  defect  of  the  Right 

Superior  Rectus.    Being  a  chart  of  objective  appearances,  the  patient  is  supposed  to  be 

behind  it,  so  that  R  and  L  are  reversed. 

doors,  though  not  so  excellent  a  one.  Figs.  78,  86  and  88  were 
taken  in  this  way  by  a  skilful  amateur.  Since  their  publication  I 
found  that  Gullstrand,  in  1896,  employed  circular  disks  for  the 
photographic  investigation  of  the  shape  of  the  cornea,  on  the 
principle  of  Placido's  disk  ;  so  that  the  idea  is  not,  in  every  part  of 
it,  a  new  one. 

Indoors,  in  the  absence  of  a  mirror,  a  small  incandescent  lamp 
may  be  found  convenient  if  fixed  just  over  the  lens,  the  patient 
being  made  to  fix  a  point  mid-way  between  the  center  of  the  lens 
and  the  center  of  the  lamp.  An  acetylene  flame  is  very  suitable. 
But  with  a  mirror  no  artificial  light  is  needed  at  all. 

Though  a  complete  photographic  record  of  an  ocular  paralysis 
would  require  nine  photographs,  yet  for  paralyses  of  single  muscles 


212  Tests  and  Studies  of  the  Ocular  Muscles 

three  amply  suffice  :  the  first,  with  the  head  set  in  the  favorite 
attitude  ;  the  second,  with  the  eyes  brought  into  the  area  of  maxi- 
mum diplopia  ;  and  the  third,  in  the  area  on  the  same  level'  as  the 
last,  but  on  the  opposite  side  of  the  median  plane.  In  each  case  it 
is  only  the  head  that  is  altered,  since  the  eyes  are  made  to  fix  the 
center  of  the  lens  always. 

Recording;  Reflections. — Fig.  89  shows  a  simple  plan  of  record- 
ing the  corneal  reflections  of  a  case  of  paralysis,  this  being,  indeed, 
a  record,  before  operation,  of  the  same  case  as  Fig.  83.  In  the 
"left  superior"  and  "left  external"  areas  the  reflections  are 
normal,  showing  that  single  vision  exists  on  looking  in  these 
directions.  The  "left  inferior"  area  shows  slight  depression  and 
adduction  of  the  cornea.  The  median  areas  show  depression 
increasing  on  looking  up,  combined  with  abduction  in  the  superior 
median  area,  adduction  in  the  primary  area  and  still  greater  adduc- 
tion in  the  inferior  median.  The  right  areas  exhibit  the  same 
features  in  a  more  marked  degree. 

An  extremely  ingenious  use  of  the  corneal  reflection  has  been 
made  by  Lucien  Howe,  who  has  used  it  to  determine  by  graphic 
methods  the  actual  rate  of  movement  of  the  eyeball  in  glancing 
from  one  object  to  another,  the  record  being  made  on  a  revolving 
sensitized  cylinder  timed  by  a  tuning  fork.  His  photographs  thus 
taken  appear  to  show  that  in  glancing  forty  degrees  the  eye  takes 
from  one-tenth  to  one-twentieth  of  a  second.  This  experimental 
method  might  with  great  advantage  be  applied  to  nystagmus. 


CHAPTER   XII 


Heterophoria  * 

Latent  deviations  of  the  eyes  involve  the  same  principles  as  do 
the  manifest  deviations  which  we  recognize  as  squints.  They  differ 
in  being  small  enough  to  come  within  the  overcoming  power  of  the 
love  of  single  vision,  but  are  liberated  from  this  superior  influence 
in  the  dark  ;  or,  when  the  vision  of  the  two  eyes  is  dissociated,  by 
making  single  vision  either  impossible  (prisms,  etc.)  or  undesired 
(glass  rod,  etc.). 

Chief  Divisions. — Latent  squints  are,  like  manifest  squints, 
grouped  into  "paralytic"  and  "concomitant"  according  or  not  as 
they  steadily  increase  on  looking  in  special  directions. 

Dissociation  of  the  Eyes.— The  demonstration  of  suppressed 
deviation  depends  on  exclusion  of  one  eye  or  on  artificial  diplopia 
of  some  kind,  so  as  to  "dissociate"  the  eyes;  or  else  on  the 
arrangement  of  two  objects,  so  that  each  is  only  seen  by  one  eye. 

By  "dissociating"  the  eyes,  we  do  not,  of  course,  mean  that 
any  of  the  innervations  are  made  to  cease  to  be  conjugate,  but 
merely  that  the  desire  for  single  vision  is  removed  so  that  the  eyes 
fall  into  their  "position  of  equilibrium." 

Thus,  if  a  strong  prism,  with  its  apex  upwards,  be  held  before  one 
eye,  everything  appears  double,  and  the  distance  between  the  double 
images  of  any  object  as,  e.  g. ,  a  candle,  is  so  great  that  the  cerebral 
centers  concerned,  utterly  unaccustomed  to  so  great  a  separation 
between  the  images  of  a  single  object,  make  little  or  no  attempt  to 
unite  them.  The  eyes  are  now  said  to  be  "dissociated,"  and  if 
any  latent  deviation  exist  it  will  express  itself  by  a  movement  of 
one  image  to  the  right  or  left  of  an  imaginary  vertical  line  passing 
through  the  other. 

This  device  for  dissociation,  as  introduced  by  v.  Graefe,  was 
for  many  years  a  favorite.  It  requires  considerable  care  in  the 
adjustment  of  the  prism,  for  since  the  image  seen  through  the 
prism  is  displaced  in  the  direction  of  its  apex,  it  follows  that  if  the 
apex  is  not  exactly  vertical,  neither  will  the  said  image  be  situated 
vertically  over  its  fellow  if  no  deviation  exist.  Thus,  a  lateral 

*  Heterophoria  is  Stevens'  name  for  latent  deviations. 

(213} 


214  Tests  and  Studies  of  the  Ocular  Muscles 

displacement  of  the  image  due  to  a  badly-placed  pri^m  may  be 
wrongly  attributed  to  the  eye.  Since,  to  begin  with,  prisms  are 
often  incorrectly  marked,  precautions  have  to  be  taken  accordingly 
and  the  prism  be  set  in  the  trial  frame  so  that  a  vertical  line  seen 
through  it  appears  unbroken  throughout. 

Physiological  Heterophoria.— Within  certain  limits  suppressed 
deviations  are  physiological,  for  though  the  accommodating  and 
converging  centers  are  functionally  connected  in  a  very  intimate 
way,  they  are  not  indivisibly  one.  Exclusion  of  one  eye,  while 
the  other  is  engaged  in  distant  vision,  causes  the  excluded  eye 
generally  to  deviate  little,  if  at  all,  inwards  or  outwards  from  its 
former  position.  As  the  object  fixed  is  brought  nearer,  the  eyes 
converge  less  than  they  accommodate  with  each  approach.  In 
consequence  of  this  we  find  that,  if  the  excluded  eye  deviates  out- 
wards in  distant  vision,  the  deviation  increases  more  and  more  as 
vision  becomes  nearer.*  By  the  time  the  object  is  within  a  foot  of 
the  eyes,  the  deviation  has  increased  to  3°  or  4°,  in  the  majority  of 
people. 

It  follows  from  this  that  in  ordinary  close  work  we  habitually 
suppress,  by  our  desire  for  single  vision,  a  deviation  to  this  extent. 
Were  the  same  deviation  to  exist  with  both  eyes  uncovered,  we 
would,  of  course,  see  double  ;  but  the  love  of  single  vision  will  not 
allow  it  to  exist  under  those  circumstances. 

The  unconscious  effort  required  to  suppress  a  deviation  of 
normal  amount  in  the  interests  of  single  vision  is  so  slight  that  we 
experience  no  inconvenience  from  it.  But  if,  from  any  cause,  the 
deviation  be  a  great  one,  the  effort  demanded  may  be  sufficient 
to  occasion  headache  or  asthenopia,  and  the  more  so  if  there 
be  any  debility  of  the  system.  Indeed,  the  effort  may  at 
times  be  given  up,  double  vision  being  accepted  until  the  tired 
centers  have  had  time  to  recuperate  themselves  for  a  fresh 
attempt  at  single  vision.  One  form  of  ' '  periodic  strabismus  ' '  is 
of  this  nature. 

Direction  Of  Deviation. — An  excluded  eye  may  deviate  in  any 
direction,  upwards,  downwards,  inwards  or  outwards,  or  in  interme- 
diate directions  ;  but  in  the  last  case  we  think  of  the  horizontal 
element  and  the  vertical  element  separately.  Since  the  power  of 
elevating  or  depressing  one  eye  above  or  below  the  other  (monocu- 
lar sursumduction  or  deorsumduction)  has  much  smaller  physio- 

*Syme  "  Fellowship  Essay,"  1882,  and  "Trans.  Oph.  Soc.,"  1883,  p.  290. 


Helerophoria  2  x  c 

logical  limits  than  horizontal  powers  of  adjustment,  it  follows  that 
vertical  deviations  are  of  so  much  the  more  importance  than 
horizontal. 

Hyperphoria. — Since,  in  concomitant  vertical  deviations  we  can- 
not tell  which  eye  is  at  fault,  the  convenient  name  "  hyperphoria  " 
was  introduced  by  Stevens.  Instead  of  speaking  of  an  upward 
or  downward  deviation  of  one  eye,  which  would  give  us  two  things 
to  remember,  viz.,  which  eye  and  which  direction,  he  speaks  of  a 
right  or  left  hyperphoria,  which  gives  us  only  one  thing  to  remem- 
ber. Thus,  if  the  right  eye  deviate  downwards  on  exclusion,  he 
calls  it  a  left  hyperphoria. 

\Ye  need,  however,  to  always  test  both  eyes  to  assure  ourselves 
that  the  deviation,  even  if  it  seem  concomitant  at  first,  is  of  the 
ordinary  kind,  for  during  an  extended  investigation  made  for  Mr. 
Berry  among  the  patients  attending  his  out-patient  department, 
several  cases  were  found  in  which  each  eye  deviated  upwards  on 
exclusion.  And,  besides  this,  the  use  of  the  term  hyperphoria 
should  not  make  us  forget  that  the  deviation  may  be  paretic  and 
due  to  actual  weakness  of  a  muscle,  causing  the  separation  of 
double  images  to  be  greater  in  some  directions,  and  to  be  greater 
when  the  paretic  eye  fixes  (answering  to  the  secondary  deviation) 
than  when  the  sound  eye  fixes  (answering  to  the  primary  deviation). 

The  following  are  the  best  tests  for  latent  deviations  : 

(i)  The  Objective  Screen  Test.— Make  the  patient  fix  a  very 
definite  test  object,  near  or  distant,  as  may  be  desired.  Screen  one 
eye  for  fully  half  a  minute.  Suddenly  withdraw  the  screen,  watch- 
ing if  the  eye  makes  an  instantaneous  movement  of  recovery 
("corrective  movement"),  and  if  so,  in  what  direction. 

A  corrective  movement  inwards  would  show  that  the  eye  had 
wandered  outwards  under  the  screen,  so  as  to  betray  a  latent 
divergence  (exophoria*}. 

A  similar  corrective  movement  outwards  would  betray  a  latent 
convergence  (esop/ioria*). 

Vertical  corrective  movements  show  that  one  eye  has  latent 
elevation  (hyperphoria)  or  latent  depression. 

These  tests  can  be  made  a  little  more  delicate  by  employing  a 
flame  for  the  patient  to  look  at,  or  by  throwing  light  on  one  eye, 
before  covering  it,  from  the  mirror  of  the  ophthalmoscope.  On 
momentary  unscreening  of  the  eye  the  position  of  the  corneal 


*  Dr.  Stevens'  terms. 


216  Tests  and  Studies  of  the  Ocular  Muscles 

reflection  can  be  observed  before  the  eye  has  had  time  to  recover 
itself. 

Graefe  recommended  that  both  eyes  be  shut  for  a  little  while 
befoie  making  any  test  for  latent  deviation. 

(2)  Subjective  Screen  Test. — Alfred  Graefe  pointed    out  that 
when  one  eye   has  a   manifest  deviation,    however  small,   sudden 
screening  of  the  fixing  eye  makes  a  candle  flame  appear  to   the 
patient  to  move,  for  the  deviating  eye  makes  a  corrective  move- 
ment in  order  to  take  up  fixation,  and  this  movement  betrays  itself 
by  an  apparent  displacement  of  its  field  of  vision. 

The  same  principle  has  been  applied  to  latent  deviations  by 
Duane,  who  has  called  it  the  "parallax  test."  After  covering  one 
eye  for  a  time,  he  suddenly  transfers  the  screen  to  the  other.  If 
the  first  eye  wandered  under  the  screen,  the  deviation  is  betrayed 
by  a  sudden  apparent  displacement  of  the  candle  in  the  opposite 
direction  to  that  in  which  the  eye  deviated. 

Thus,  if  the  right  eye  be  the  one  first  screened  and  the  flame  moves  to 
the  right,  there  is  esophoria  ;  but  if  the  flame  moves  to  the  left,  exophoria. 
Duane  calls  them  respectively  "homonymous  parallax"  (P//)  and  "crossed 
parallax"  (PX).  If  the  flame  moves  down,  he  calls  it  "right  parallax" 
(PR),  because  it  shows  that  the  right  eye  deviates  upwards  (right  hyper- 
phoria)  ;  and  if  the  flame  moves  up,  "left  parallax"  (PL),  showing  left 
hyperphoria. 

This  test  presents  the  advantage  of  requiring  no  apparatus. 
Otherwise,  its  usefulness  is  doubtful  except  for  a  skilled  patient. 
Parallax  "can  be  measured  in  terms  of  the  prism  which  causes  its 
abolition."  For  near  vision,  a  dot  on  a  piece  of  paper  replaces 
the  flame. 

(3)  Prism  Tests. — These  were  introduced  by  von  Graefe  long 
ago.     A  prism,  with  its  base  up  or  down,  strong  enough  to  pro- 
duce insuperable  vertical  diplopia,  was  held  before  one  eye,  while 
the  patient   looked  either  at    a    distant   flame    or    at  a  dot    on  a 
card  (with  a  vertical   line  through  it,  which,  however,  it  is  better 
without). 

If  one  image  appeared  to  wander  to  the  right  or  left  of  the 
other,  the  eye  to  which  it  belonged  was  proved  to  have  deviated  in 
the  opposite  sense. 

The  amount  of  deviation  was  measured  by  the  prism,  base  in 
or  out,  required  to  bring  the  two  images  again  into  one  vertical 
line. 


Heterophoria 


217 


The  difficulty  of  ensuring  that  a  prism  is  strictly  base  up  or 
down,  and  the  considerable  inaccuracies  introduced  by  even  slight 
departures    from  a  correct    position,   led    the    writer    to   design    a 
"double  prism,"  shown  in  Fig.  90, 
like  two  thin  prisms  joined  at  their 
bases  but  made  of  one  piece  of  glass 
and     somewhat     similar,     therefore, 
to    the    bi-prism    used    by    Fresnel 
to  demonstrate  phenomena  of  inter- 
ference. 

When  this  is  held  before  one 
eye  of  the  patient  he  sees  with  that 
eye  two  images  of  a  flame,  one  above 
and  the  other  below,  the  real  image 
seen  by  the  naked  eye.  When  the 
two  false  images  are  vertical,  the 

prism  is  correctly  held  and  it  is  easy  to  judge  whether  the  third 
and  real  image  lies  to  the  right  or  left  of  the  imaginary  line 
between  them. 

Dr.  Stevens,  of  New  York,  feeling  the  same  difficulty  about 
Graefe's  prism,  overcame  it  in  a  different  way — by  his  excellent 
and  well-known  "  phorometer,"  in  which  prisms  are  mechanically 


Fig.  yo 

Double  Prism  (square  form) 


FIR. 


<H 


Side  view  of  a  Double  Prism.     (From  "  Journ.  Anat.  and  I'hys.,"  vol  xx,  p.  496) 


geared  together  in  a  stand,  as  shown  in  Fig.  92.  Numerous 
phorometers  have  since  been  made  by  others,  many  of  them  of 
excellence  (e.g.,  Prince's.  Risley's,  etc.).  Most  of  these,  how- 
ever, are  expensive  and  possess  no  advantage  over  the  simpler  glass 
rods,  to  be  next  described. 


218 


Tests  and  Studies  of  the  Ocular  Muscles 


It  should,  perhaps,  be  noticed  that  in  all  tests  in  which  the 
eyes  are  dissociated  by  the  prisms,  the  desire  for  uniting  the  double 
images  is  not  wholly  abrogated,  but  only  the  power  to  do  so  taken 
away.  Unless,  therefore,  the  separation  of  the  two  images  be  con- 
siderable, there  is  a  tendency  to  bring  them  into  one  vertical  line. 
This  tendency,  however,  if  a  strong  prism  be  used,  is  quite  slight 
and  insignificant,  and  is,  I  think,  removed  altogether  if  the  images 
are  made  quite  dissimilar,  as  in  the  next  test. 

(4)  The  Glass-Rod  Test  and  its  Allies.— Owing  to  imperfect 
manufacture  of  the  double  prism,  described  in  the  last  section,  the 


The  Prisms  of  Stevens'  Phorometer 


ridge  had  a  rounded  instead  of  a  sharp  edge,  and  this  caused  the 
appearance  of  a  faint  band  of  light  *  joining  the  two  false  images. 
It  was  this  band  of  light  which  led  me  to  employ  a  glass  rod  to  pro- 
duce a  more  pronounced  streak  of  light.  This  simple  "rod  test" 
does  not,  like  the  prism  tests,  depend  on  separation  of  the  two  images 
of  an  object,  but  on  alteration  of  the  shape  of  one  of  the  images  so 
that  it  is  no  longer  recognized  as  belonging  to  the  same  object. 

Optical  Explanation. — When  a  point  of  light  is  viewed  through 
a  transparent  cylinder  it  appears  drawn  out  into  a  line  of  light  at 
right  angles  to  the  axis  of  the  cylinder,  for  the  simple  reason  that 

*Mr.  Berry  noticed  this  band  was  transversely  striated,  and  showed  the  striations  to  be 
many  of  them  due  to  the  phenomenon  o<"  "  interference."  If  Mr  Berry  had  not  taken  so 
much  interest  in  this  band.  I  might  not  have  attended  to  it  sufficiently  to  have  thought  of  the 
possible  utility  of  it  and  the  reproduction  of  it  by  a  glass  rod. 


Hetcrophoria  219 

the  radiant  point  is  put  out  of  focus  for  the  letina  in  this  meridian 
but  not  in  that  which  corresponds  to  the  axis. 

It  forms,  in  fact,  a  "  diffusion  line"  ;  just  as  a  strong  spherical 
lens  by  acting  equally  in  all  meridians  forms  a  "diffusion  circle." 

The  whole  light  which  emerges  from  the  cylinder  thins  down 
like  a  wedge  into  a  line,  parallel  to  the  axis,  from  which  it  again 
spreads  out  on  to  the  cornea  to  be  collected  finally  into  a  line,  per- 
pendicular to  the  former  line,  on  the  retina.  The  length  of  the  diffu- 
sion line  thus  thrown  on  the  retina  is  exactly  that  of  the  diameter  of 
the  diffusion  circle  created  by  a  spherical  lens  of  the  same  power. 

If  the  other  eye,  which  has  no  rod  before  it,  be  open,  the 
flame  is  seen  by  it  nakedly,  and  since  a  line  and  a  flame  are  not 
mentally  conceived  to  be  double  images  of  the  same  object,  the 
desire  to  unite  them  is  quenched  for  the  time,  and  all  the  more  so 
if  they  are  differently  colored  by  making  the  rod  of  red  glass  and 
holding  a  plane  blue  or  green  disk  before  the  other  eye. 

Two  Species  Of  Cylinder. — Since  a  long  line  is  a  desideratum, 
we  must  use  a  strong  cylinder  to  produce  it.  The  form  of  cylinder 
at  first  used  was  a  simple  glass  rod,  to  be  found  almost  everywhere, 
such  as  a  stirring  rod,  thermometer  tube,  etc.,  whence  the  name  of 
"  rod  test"  was  that  originally  employed. 

I  found,  later,  by  experiment,  that  a  more  perfect  line  and  a 
greater  conservation  of  light  is  obtained  by  a  piano-cylinder  of 
such  a  strength  that  when  held  as  close  to  the  eye  as  practicable, 
the  principal  focus  shall  lie  in  the  plane  of  the  pupil.  It  differs 
from  the  glass  rod  in  that  the  light  which  traverses  it  is  focused 
within  the  eye  only,  without  first  being  narrowed  up  into  a  line 
before  reaching  the  cornea. 

This  cylindrical  lens,  already  described,  though  optically  more 
perfect,  is  not  nearly  so  useful  clinically  as  a  series  of  mounted  rods. 
A  plano-convex  cylindrical  lens*  with  a  radius  of  curvature  of  about 
10  mm.  is  the  one  which  when  held  at  the  usual  distance  from  the 
eye,  admits  the  maximum  of  light  through  the  pupil. 

With  such  a  lens,  so  little  light  is  lost  that  even  the  stars  can 
be  used  to  test  the  equilibrium  of  the  eyes.  Immense  distances, 
however,  seem  to  give  the  same  results  as  moderate  ones. 

The  line  produced  by  a  star  is  of  exquisite  delicacy.  While 
this  form  of  cylinder  gives  a  brighter  and  more  even  line  than  any 

*~In  my  first  description  of  this  lens  ("Oph.  Review,"  vol.  xii,  p.  40)  for  "radius  of  curva- 
ture" read  "focal  length."  The  actual  lens  I  had  been  using  had  a  radius  of  curvature  of 
10  ram.  and  a  focal  length,  therefore,  of  about  20  min. 


22O  Tests  and  Studies  of  the  Ocular  Muscles 

other,  it  needs  more  careful  placing  before  the  pupil  than  the  group 
of  glass  rods  already  described,  and  this  spoils  it  for  rapid  clinical 
work.  Swan  Burnett  suggested  a  weaker  cylinder  still,  moved  up 
and  down  before  the  eye  to  compensate  for  the  shortness  of  the 
line,  and  in  the  absence  of  a  special  instrument,  a  cylindrical  lens 
from  the  trial  case  can  be  thus  utilized. 


a  Fig.  93  b 

(a)  The  first  form  of  the  Glass  Rod  Test :  now  converted  into  (b)  a  row  of  glass  rods. 

Manufacture. — The  rod  should  be  free  from  flaws  and  from 
tapering,  and  to  prevent  the  flame  being  seen  past  its  edges,  if 
only  a  single  rod  be  used,  it  should  be  mounted  in  a  slit  cut  out  of 
the  central  part  of  a  circular  metal  or  vulcanite  disk.  This  is  shown 
in  Fig.  93,  the  use  of  the  disk  being  to  exclude  all  light  from 
entering  the  eye  except  that  which  traverses  the  glass  cylinder. 

Probably  the  best  form  of  instrument  is  the  modification  sug- 
gested by  Mr.  Berry,  viz. ,  a  piece  of  plane  glass  corrugated  or 
grooved  with  rounded  linear  elevations  and  depressions  ;  but, 
owing  to  its  difficulty  of  manufacture,  I  chose  rather  to  fix  together 
a  number  of  small  glass  rods  side  by  side.  When  this  is  carefully 
done,  so  that  the  line  of  light  produced  by  each  rod  forms  part  of 
the  whole  line  without  distortion,  we  obtain  a  longer  total  line  of 
light  and  require  the  exercise  of  less  care  in  placing  the  apparatus 
before  the  pupil.  The  superiority  of  the  multiple  rods  over  the 
single  rod  is  at  once  evident  on  using  them. 

It  is  easy,  after  laying  a  row  of  short  rods  side  by  side  on  a 
smooth  surface,  such  as  that  of  a  mantelpiece,  to  cement  their  ends 
together  with  sealing  wax.  For  good  work,  however,  they  should 
be  mounted  in  a  metal  or  vulcanite  disk.* 

*The  diameter  of  the  disk  to  which  the  axes  of  the  rods  are  parallel  should  be  carefully 
marked  upon  it,  and  also  the  diameter  at  right  angles  to  this. 


Heterophoria 


221 


Mode  Of  Use. — Whatever  form  of  rod  or  cylinder  be  employed 
(since  the  line  of  light  is  always,  of  necessity,  at  right  angles  to 
the  axis),  if  we  wish  to  produce  a  vertical  streak  (with  which  to 
test  horizontal  deviations)  the  axis  must  be  held  horizontally,  and 
to  produce  a  horizontal  streak  (with  which  to  test  vertical  devia- 
tions) it  must  be  held  vertically. 

Though  not  essential,  the  test  is  greatly  improved  by  using 
rods  of  red  glass  and  holding  a  piece  of  green  or  blue  glass  before 
the  other  eye.  This  makes  the  streak  and  the  flame  differently 
tinted  and  also  makes  the  streak  more  visible  by  subduing  the 
excessive  brilliancy  of  the  flame. 

Since  the  streak  of  light  is  of  exactly  the  same  breadth  as  the 
flame,  a  candle  gives  a  good  vertical  band,  but  a  feebler  and  broader 
horizontal  one.  When  we  test  for  vertical  deviations,  therefore,  it 
is  better  either  to  use,  as  I  do,  a  miniature  lamp,  or  else  a  gas  jet 
turned  low  and  not  more  than  a  quarter  of  an  inch  in  height.  A 
small  toy  paraffine  lamp  answers  beauti- 
fully, especially  with  black  velvet  behind  it. 

The  patient  should  be  placed  at  a  dis- 
tance of  not  less  than  5  or  6  meters  from 
the  flame.  Distances  greater  than  this, 
even  ten  or  twenty  miles  away,  as  when 
a  lighthouse  is  viewed  at  sea,  give  prac- 
tically the  same  results. 

We  begin  by  holding  the  rod  hori- 
zontally to  test  for  a  horizontal  deviation. 
If  the  line  of  light  appear  to  pass  through 
the  flame,  as  (a)  in  Fig.  94,  the  balance  is 
perfect.  If  it  lie  notably  to  one  or  other 
side  of  it,  as  (3)  or  (r)  in  the  same  figure, 
a  deviation  exists  to  an  amount  propor- 
tionate to  the  distance  apart. 

If  the  line  be  to  the  same  side  of  the  flame  as  the  glass 
rod,  the  diplopia  is  evidently  direct  (i.  e.,  homonymous),  indi- 
cating latent  convergence  (esophoria);  if  to  the  other  side,  the 
diplopia  is  crossed  (;.  e.,  heteronymous)  indicating  latent  diver- 
gence (exophoria). 

To  note  the  vertical  equilibrium,  we  next  rotate  the  rod  into 
the  vertical  position,  which  produces  a  horizontal  line  of  light. 
In  the  vast  majority  of  patients  this  line  will  be  found  to  pass 


F  g.  94 

Use  of  R<xl  Test. 


222 


Tests  and  Studies  of  the  Ocular  Muscles 


almost  exactly  through  the  flame.  Should  it  lie  above  the  flame, 
it  indicates  a  downward  deviation  of  the  eye  before  which  the 
glass  rod  is  held  ;  but  if  below  the  flame,  it  indicates  an  upward 
deviation  of  the  eye.  It  is  easy  to  remember  the  old  rule,  that 
the  deviation  of  an  eye  is  in  the  opposite  direction  to  that  of 
its  false  image,  which  applies  to  both  vertical  and  horizontal 
deviations.* 

Measurement  of  Latent  Deviations. — (i)  The  extent  of  a 
deviation  may,  in  the  absence  of  a  scale,  be  roughly  estimated 
by  rinding  at  what  horizontal  or  vertical  distance  from  the  flame 

an  object  may  be  placed  on  the 
wall  to  be  crossed  by  the  streak 
of  light. 

(2)  If  a  tape  measure  coiled 
up  in  a  metal  box  be  available,  a 
very  good  way  is  to  hold  the  zero 
of  the  scale  just  behind  the  flame 
and  draw  out  the  box  till  it  meets 
the  line  of  light. 

This  method,  like  the  last, 
however,  is  subject  to  the  draw- 
back that,  to  leave  the  observer 
free  to  make  the  measurement, 
the  rod  must  be  placed  in  a  trial  frame  or  be  held  by  the  patient 
himself.  We  need  only  divide  the  number  of  centimeters  recorded 
on  the  tape  by  the  number  of  meters  the  patient  stands  away  from 
the  flame,  to  get  the  deviation  of  the  eye  expressed  in  prism  diopters. 
To  convert  this  approximately  into  degrees,  multiply  by  4  and  divide 
by  7,  since  a  prism  diopter  is  four-sevenths  of  a  degree.  Since  each 
succeeding  prism  diopter  has  a  smaller  value  than  the  last,  the  unit 
is  a  bad  one  for  high  deviations.  The  degree  is  still  the  best  unit, 
unless,  perhaps,  for  the  decentering  of  lenses. 

*A  Rare  Anomaly.— I  have  noticed  in  some  rare  cases  of  strabismus  that  the  deviation 
of  an  eye  which  is  totally  excluded  from  vision  is  different  from  its  deviation  when  its  exclu- 
sion is  not  total,  even  though  the  deviating  eye  takes  no  share  in  fixation.  In  addition, 
therefore,  to  placing  the  rod  before  one  eye,  it  is  sometimes  well  to  exclude  that  eye  and 
expose  it  moment  aril  y,  that  the  line  of  light  may  be  seen  in  a  position  answering  to  the  devia- 
tion on  exclusion.  It  is  easy  to  do  this  by  holding  one  side  of  the  metal  disk  which  bears  the 
rods  before  the  pupil  and  then  steadily  moving  the  rod  across  the  line  of  vision  till  the  pupil  is 
again  covered  by  the  other  side  of  the  disk.  The  streak  is  thus  visible  for  a  moment  and  is 
again  extinguished  before  the  eye  has  time  to  redress  itself.  The  result  of  this  test  in  ordinary 
cases  will  nearly,  if  indeed  not  quite,  always  be  found  to  coincide  with  that  obtained  by  keep- 
ing the  rod  always  before  the  pupil,  showing  that  in  the  vast  majority  of  cases  the  ordinary 
use  of  the  rod  reveals  as  much  of  the  latent  conditions  as  complete  exclusion  would  do,  and 
this  is  still  further  ensured  if  the  recommendation  to  hold  a  colored  glass  before  the  other  eye 
be  observed. 


Fig 


Tape  marked  in  Tangents  of  Degrees,  as  well 
as  in  Centimeters 


Heterophoria 


223 


(3)  When  the  patient  has  a  visual  acuity  too  poor  to  read  the 
figures  on  a  tangent  scale,  prisms    may  be  used    to  measure  the 
deviation,  that  prism  which  brings  the  streak  and  the  flame  together 
(provided  it  be  held  with  its  base-apex  line  perpendicular  to  the 
streak)  being  the  one  whose  deviating  angle  expresses  the  devia- 
tion.     Many  ingenious    prismatic   contrivances,   the  description  oi 
which  belongs  to  another  work,  have  been  made  to  dispense  with  a 
number  of  prisms  ;  and  some  of  these,  such  as  Prince's  phorometer, 
Jackson's  triple  rotary  prism  and  Risley's  prisms,  have  been  com- 
bined with  the  rod  test  and  do  their  work  well. 

(4)  Tangent  scales,  for  permanent  fixtures  on  the  wall  of  the 
consulting  room,  afford  the  method  which  I  prefer  to  any  other. 
These  scales  are  now  constructed  both  for  horizontal  and  vertical 
deviations,  as  shown  at  Fig.  96.     They  are  graduated  for  use  at 
5  meters  and  are  marked  with  large  figures  representing  degrees  *  of 
deviation,  these  figures  being  in  black  to  the  right  and  in  red  to  the 
left  of  the  scale.      The  zero  is  at  the  center,  and  there  a  tiny  paraffin 
lamp  is  fixed  with  a  small  piece  of  black  velvet  behind  it,  to  heighten 


Tig.  96 

To  illustrate  the  use  of  the  author's  Tangent  Scales,  the  streak  of  light  being  represented  by 

SS,  which  (if  the  patient  be  5  m.  distant)  reveals  3%°  of  hyperphona  in  the  naked  eye. 

The  naked  eye  sees  the  scales  only  :  the  rod-clud  eye  sees  the  streak  only 

its  effect  by  contrast ;  the  apparent  brilliancy  thus  attained  is  striking. 

It  is  a  good  plan  to  have  the  horizontal  and  vertical  scales, 

both  in  position  at  right  angles  to  each  other,  with  zeroes  coinciding. 

*  Those  who  use  prism  diopters  can  easily  add  small  figures  at  intervals  of  5  cm  to  the 
ccale  for  their  own  perusal. 


224  Tests  and  Studies  of  the  Ocular  Muscles 

The  horizontal  element  of  a  deviation  should  be  recorded  first,  and 
then  the  vertical  element. 

Previous  Tangent  Scales. — Tangent  scales  for  the  measurement 
of  strabismus  were  introduced  by  Landolt  and  Hirschberg  about 
the  same  time  (1875).  Landolt' s  consisted  of  a  tape  marked  at 
intervals  of  5°,  for  extending  along  three  walls  of  the  consulting 
room,  the  patient  standing  at  a  point  equally  distant  from  the  three. 
Another  vertical  tape  extended  from  the  floor  to  the  ceiling,  and 
along  the  floor  also.  This  was  an  excellent  plan,  enabling,  by  a 
candle  and  a  piece  of  glass,  the  squint  to  be  measured  with  definite 
obliquities  of  vision.  Hirschberg' s  would,  perhaps,  be  better 
described  as  a  tangent  map,  and  served  its  purpose  excellently  as  a 
strabismometer.*  There  is  nothing  new,  therefore,  in  the  tangent 
scale  of  Fig.  96,  except  the  large  and  legible  figure  intended  for 
the  patient  to  read  as  well  as  the  surgeon. 

Advantages  of  the  Tangent  Scale. — When  the  patient  has 
sufficient  acuity  of  vision  to  read  the  large  printed  figures,  on  a 
scale  affixed  to  the  wall,  this  mode  of  measurement  surpasses  any 
other,  since  it  is  not  only  free  from  the  fallacies  of  incorrectly-placed 
prisms,  but  it  enables  the  patient  to  read  off  the  amount  of  devia- 
tion from  moment  to  moment,  as  it  wavers  or  as  it  gradually 
increases  or  decreases,  by  which  we  gain  some  information  as  to 
the  nature  of  the  deviation. 

This  information  is  difficult  to  obtain  if  we  dispense  with  the 
scale  and  only  measure  the  deviation  by  the  prism  which  annuls  it. 

Yet  another,  and  perhaps  the  chief,  point  in  favor  of  the  scale 
is  that  it  allows  the  surgeon  to  check  the  accuracy  of  the  patient 
by  observing  whether  the  position  of  the  streak  is  altered  by  a 
prism  held  before  the  patient's  eye  to  the  correct  amount  of  its 
own  deviating  angle.  Thus,  if  a  deviation  of  -f-  3°  be  reported  by 
the  patient  (i.  e. ,  if  the  streak  cross  the  black  figure  3  on  the  same 
side  of  the  flame  as  the  eye  before  which  the  rods  are  held)  a  prism 
of,  say,  8°  deviation  held  before  the  naked  eye,  base  out,  would 
change  the  streak  to  the  red  figure  5.  If  the  patient  reports  this 
truly,  he  can  be  trusted  for  the  rest  of  the  examination,  f 

It  is,  however,  but  rarely  that  such  precautions  are  necessary, 

*"Aunales  d'Oculistique,"  1875;  "Brit.  Med.  Journ.,"  Jan.  1,  1881. 

fThe  rare  and  only  fallacy  in  using  a  prism  to  shift  the  streak  of  light  is  that  if  the  prism 
he  too  strong  it  may  impair  the  visual  acuity  of  the  eye,  hefore  which  it  is  held,  enough  to 
transfer  fixation  to  the  other  eye.  In  most  cases,  even  should  this  occur,  the  readings  would 
be  accurate;  hut  in  anisometropia  or  in  paralytic  muscular  affections,  the  readings  may  be 
greater  during  fixation  with  one  eye  than  during  fixation  with  the  other.  Such  conditions, 
however,  are  detected  by  their  own  appropriate  tests. 


Heterophoria  225 

and  it  generally  suffices,  if  the  patient  be  doubted,  to  transfer  the 
disk  from  one  eye  to  the  other,  to  find  if  the  streak  is  said  to  cross 
similar  figures  on  opposite  sides  of  the  flame.  It  will  be  seen, 
therefore,  that  the  glass  rod  is  clear  from  the  opprobrium  of  most 
subjective  tests  that  they  leave  us  at  the  mercy  of  the  patient's 
statements. 

The  reason  that  figures  to  the  right  of  the  flame  are  black  and 
those  to  the  left  red,  is  to  give  the  patient  a  double  mode  of  describ- 
ing to  which  side  of  the  candle  the  streak  lies.  I  generally 

(a)  Ask  which  side  of  the  flame  the  streak  is  :  right  or  left? 
(If  a  child  it  is  useful  to  touch  its  right  or  left  hand  or  shoulder  in 
company  with  the  words. ) 

(b)  What  figure  does  it  pass  through  ? 

(c)  Is  the  figure  black  or  red  ? 

(X)  If  needful,  transfer  the  rod  to  the  other  eye,  and  ask  the 
same  questions. 

The  same  order  applies  to  the  vertical  scale,  substituting 
"above"  and  "below"  for  "right"  and  "left/' 

Which  Eye  Fixes? — It  is  sometimes  a  question  whether  the 
patient  is  fixing  the  flame  or  the  streak.  Though  he  cannot  fix 
both  at  the  same  time,  he  is  free  to  fix  either,  just  as  in  Graefe's 
prism  test,  the  patient  can  at  will  fix  either  the  naked  image  or  the 
prismatic  one,  either  eye  being  able  to  move  so  as  to  receive  an 
image  on  the  fixing  point  of  its  retina,  though  the  movement  dis- 
places the  fixing  point  of  the  other  eye  away  from  its  image. 

If  alternate  fixation  makes  the  streak  shift  to  different  figures, 
the  case  is  one  of  either  anisometropia  or  paresis.  Alternate 
fixation  can  generally  be  secured  by  transferring  the  rods  from 
one  eye  to  the  other,  and  so  delicate  a  revealer  of  anisometropia  is 
this  procedure  sometimes  that  so  small  a  difference  as  .25  D.  was 
once  detected  (before  the  refraction  was  tried)  in  a  person  with  one 
eye  Em.,  and  the  other  H.  —  .25  D.  Such  cases  show  that  the 
relation  between  accommodation  and  convergence,  though  very 
trainable  is  very  delicate  too. 

Other  Uses  Of  the  Scale. — Since  multiplication  of  apparatus  is 
undesirable,  it  may  be  said  in  passing  that  the  same  scale  serves  to 
measure  the  deviating  angle  of  prisms,  to  measure  squints,  the 
angle  -y,  etc. 

Test  for  Concomitancy. — Heterophorias  should,  in  all  cases,  be 
tested  in  different  parts  of  the  motor  field,  to  see  if  they  are 


226  Tests  and  Studies  of  the  Ocular  Muscles 

concomitant  or  paralytic.  This  is  most  easily  done  by  repeating  the 
test  with  the  head  held  in  different  positions.  To  avoid  confusion 
of  mind,  a  good  plan  is  to  imagine  the  aerial  screen  of  Fig.  74,  to 
form  a  rigid  system  with  the  patient's  face,  so  that  when  the  face 
looks,  for  instance,  down  and  to  the  left,  the  eyes  looking  straight 
forward,  look  through  the  right  superior  area.  All  that  has  been 
said  heretofore  about  recording  ordinary  paralyses  applies  equally 
to  paralytic  latent  deviations. 

Trial  Test  for  Reliability.— Before  leaving  the  subject  of  the 
rod  test,  it  may  be  well  to  remark  that  to  secure  full  confidence  in 
their  use,  tests  for  the  eye  should  themselves  first  be  tested,  and 
nothing  is  easier  than  for  the  surgeon  to  test  the  reliability  of  the 
disk  of  rods  for  himself. 

Selecting  some  correctly- marked  prisms,  stand  precisely  five 
meters  from  the  tangent  scales,  with  the  disk  of  red  rods  before 
one  eye  and  a  green  or  blue  glass  before  the  other.  If  orthophoria 
exist,  prisms  held  truly  *  in  combination  with  colored  glass  before 
the  other  eye  (or  even  without  it)  will  make  the  red  streak  appear 
to  pass  through  figures  which  correctly  describe  their  deviating 
angles.  It  will  be  noticed,  also,  that  however  near  the  streak  is  to 
the  flame,  no  tendency  exists  for  them  to  run  together. 

HeterophOlia  in  Near  Vision. — For  near  vision  the  glass  rod  is 
not  very  suitable,  on  account  of  the  difficulty  of  obtaining  a  suf- 
ficiently fine  point  of  light.  It  is  true  that  by  utilizing  a  tiny  electric 
lamp  behind  a  hole  in  the  cardboard,  or  the  reflection  from  a  minute 
silver  button  fastened  in  the  middle  of  a  tangent  scale  on  a  sheet  of 
paper,  and  looking  at  this  through  a  piano  cylinder,  to  economize 
all  the  light  proceeding  from  it,  a  fairly  good  streak  can  be  obtained  ; 
but  for  clinical  work  I  have  abandoned  this  for  my  older  small 
"tangent  scale,"  published  in  1884,  and  fixed  just  inside  the 
cover  of  this  book. 

This  scale  is  adapted  for  use  at  a  quarter  of  a  meter  from  the 
patient's  eye  and  is  graduated  in  degrees  to  the  right  and  left  of  a 
central  zero,  the  right  hand  figures  being  black  and  the  left  hand 
red.  Meter  angles,  represented  by  Mlt  Af2,  J/3,  J/4,  are  also 
marked  on  the  scale.  From  the  central  zero  projects  a  vertical 
arrow,  and,  at  Mr.  Berry's  suggestion,  I  have  placed  a  printed 
sentence  just  below  the  figures  to  ensure  the  patient's  accommo- 

*The  way  to  hold  a  prism  truly  is  to  observe  the  image  caused  by  double  internal  reflec- 
tion, described  in  "Ophthalmological  Prisms,"  2d  edition,  p.  51,  and  which  indicates  the  apex 
with  unerring  fidelity. 


Heterophoria 


227 


dation.  The  small  figures  are  prism  diopters  for  those  who  use 
that  unit. 

The  scale  can  be  triplicated  by  using  a  double  prism,  but  the 
slightly  greater  accuracy  thus  secured  is  not  so  valuable  as  the 
saving  of  time  effected  by  employing  a  single  square  prism  of  6° 
deviation  (about  12°  old  marking,  or  10^^). 

Mode  Of  Use,— Holding  the  scale  ^  meter  before  the  patient 
(about  ten  inches),  place  the  square  prism,  edge  up,  before  his  right 
eye.  He  now  sees  two  scales  and  two  arrows.  Being  instructed  to 
look  at  the  lower  arrow  he  is  asked  what  figure  it  seems  to  shoot 
up  at,  also  whether  the  figure  is  black  or  red,  and  as  a  control  test 
he  is  asked  what  word  in  the  printed  sentence  the  arrow  points  to. 

Its  Meaning. — The  surgeon,  remembering  that  the  lower  arrow 
is  seen  by  the  left  eye  (since  the  image  seen  through  the  prism  is 
displaced  upwards  in  the  direction  oi  its  apex)  knows  at  once  the 
nature  and  amount  of  any  deviation,  for  if  the  lower  arrow  wander 
to  the  right,  diplopia  is  crossed  ;  and  if  to  the  left,  it  is  homony- 
mous,  showing  respectively,  relative  divergence  and  convergence. 
To  save  even  this  little  calculation,  however,  "divergence"  and 
"  convergence  "  are  marked  on  the  scale. 

Physiological  ExophOria. — There  is  usually  exophoria  at  ten 
inches  of  about  one  (binocular)  meter  angle,  therefore  3°  40';  but 
it  varies  from  o°  to  6°  or  8°.  Variations  at  this  distance  are  not 
nearly  so  significant  as  in  distant  vision,  unless  esophoria  is  found 
to  be  present.  It  is  difficult  to  fix  an  exact  limit  between  the 
physiological  and  the  pathological,  and  every  case  must  be  taken 
on  its  own  merits,  this  test  being  only  one  of  the  many  required  to 
thoroughly  examine  a  case. 

Prism  Diopters  in  Near- Vision  Tests. — A  fourth  of  a  centimeter 
at  25  cm.  distance  represents  one  prism  diopter,  and  it  is  quite  easy 
to  mark  a  card  in  this  unit ;  but  personally  I  prefer  the  old  degrees, 
since  it  would  be  anomalous  to  measure  manifest  squints  with  one 
unit  and  latent  squints  with  another.  Degrees  are  converted  into 
prism  diopters  when  multiplied  by  7  and  divided  by  4. 

Intermediate  Scales. — Though  I  have  constructed  scales  for 
use  at  distances  intermediate  between  5  m.  and  ^  m. ,  they  are  not 
needed  in  practice. 

Tests  for  Breadth  Of  Fusion. — By  breadth  of  fusion  is  meant 
the  measure  of  the  power  of  suppressing  in  the  interests  of  single 
vision,  artificially  created  diplopia. 


228 


Tests  and  Studies  of  the  Ocular  Muscles 


In  all  cases  of  hyperphoria  it  is  recommended  by  some  to  test 
the  vertical  breadth  of  fusion  by  finding  both  the  strongest  prism 
edge  up  and  the  strongest  prism  edge  down,  which  can  be  held 
before  the  hyperphoric  eye  without  causing  insuperable  diplopia. 
Half  the  difference  between  the  prisms  measures  the  true  hyper- 
phoria by  this  method,  provided,  of  course,  that  the  prisms  are 
numerated  according  to  their  deviation.  Thus,  a  sursumducent 
prism  (i.e. ,  edge  up)  of  2°  d.  and  a  deorsumducent  prism  (i.  e., 
edge  down)  of  ^2°  shows  a  hyperphoria  of  ^°,  with  a  vertical 
"breadth  of  fusion"  of  2*4°.  Jackson's  triple  rotary  prism  is 
very  suitable  for  this  test. 


Fig.  97 

A  convenient  Trial  Frame,  for  the  temporary  and  tentative  use  of  prisms  of  Americau  type 

Correction  Of  Hyperphoria.  —  In  using  relieving  prisms  or 
decentered  lenses  to  correct  hyperphoria,  it  is  not  well  to  correct 
quite  the  whole  of  it.  A  "relieving  prism"  should  be  placed, 
apex  up,  before  the  hyperphoric  eye  ;  or,  if  strong,  it  should  be 
divided  into  two,  one  half  being  placed  thus  before  the  hyperphoric 
eye  and  the  other  half,  apex  down,  before  the  descending  eye. 

Convex  lenses  should  be  decentered  downwards  before  the 
hyperphoric  eye  and  upwards  before  the  other  eye  ;  concave  lenses 
upwards  before  the  hyperphoric  eye  and  downwards  before  the 
other.  The  amount  of  decentering  should  be  calculated  as  fol- 
lows :  Divide  the  number  of  required  prism  diopters  by  the  num- 
ber of  diopters  in  the  lens  ;  this  gives  the  decentering  in  centimeters. 
To  express  it  algebraically  — 

A 


where  d  is  the  decentering  of  each  lens  in  centimeters,  D  the 
dioptric  strength  of  each  lens  in  the  vertical  diameter,  and  A  the 
number  of  prism  diopters  by  which  it  is  decided  to  relieve  each  eye. 
Relative  Importance  Of  Correction.  —  The  correction  of  persis- 
tent hyperphoria  is  far  more  desirable  than  that  of  any  other  form 


Heterophoria  220 

of  heterophoria  ;  next  in  importance  comes  that  of  esophoria,  and 
last  that  of  exophoria. 

Concomitancy  VS.  Paresis.— We  do  well  to  remind  ourselves 
again  in  this  connection  that  hyperphoria  should  always  be  tested 
with  the  face  thrown  in  turn  forwards  and  backwards  to  test  for 
concomitancy  on  the  one  hand  or  paresis  on  the  other  :  it  is  apt  to 
be  too  much  taken  for  granted  that  all  hyperphoriae  are  concomitant. 
If  the  diplopia  increase  on  looking  up  or  down,  it  should  be  tested 
in  the  nine  motor  areas,  as  described  in  the  chapter  on  paralyses, 
and  attention  should  be  also  paid  to  the  torsion  of  the  false  image. 

The  Horizontal  Breadth  of  Fusion.— This  is  measured  by 
prisms  with  their  apices  in  and  out. 

The  power  of  overcoming  crossed  diplopia  (produced  by 
prisms  apex  in)  constitutes  the  positive  range  of  fusion,  and  the 
power  of  overcoming  homonymous  diplopia  (produced  by  prisms 
apex  out)  constitutes  the  negative  range  of  fusion. 

A  series  of  prisms,  each  stronger  than  the  last,  may  be  used  ; 
or,  better  still,  Cretes',  Risley's  or  Jackson's  rotary  prisms.  They 
all,  however,  possess  the  disadvantage  of  having  to  be  held  before 
one  eye  and  by  impairing  the  distinctness  of  one  image,  leaving 
the  other  image  naked,  they  tend  to  lessen  the  desire  for  single 
vision.  Some  such  arrangement  as  that  which  I  have  designed 
and  shown  in  Fig.  98  is  better  where  two  prisms  have  their  apices, 


Fig:.   98 

Double  Prismatic  Triai  Frame 

to  begin  with,  both  up  or  both  down,  or  both  to  one  side,  and  are 
simultaneously  rotated  in  opposite  directions.  The  proportion  be- 
tween the  positive  and  negative  range  differs  according  to  the  dis- 
tance of  the  object  looked  at,  the  negative  range  being  small  in 
distant  vision  and  greater  in  near  vision. 


230  Tests  and  Studies  of  the  Ocular  Muscles 

Orthoptic  Training. — Javal  and  others  have  shown  how  very 
much  the  horizontal  relation  between  convergence  and  accommoda- 
tion can  be  altered  by  systematic  training,  and  when  marked  hete- 
rophoria  is  not  relieved  by  the  correction  of  refraction,  I  have  come 
to  believe  it  is  far  better  to  use  training  prisms  than  relieving  ones. 
Instead  of  supplying  the  patient  with  a  series  of  prisms  increasing 
in  strength,  I  now  employ  the  frame  of  Fig.  97  and  order  one  pair 
of  strong  prisms  to  be  set  therein.  They  can  then  be  reset  from 
time  to  time,  so  as  to  exert  a  stronger  and  stronger  action.  For 
example,  with  their  apices  down  and  in  they  would  act  as  weak  con- 
verging prisms  and  would  be  suitable  to  commence  the  training  of 
an  exophoria.  At  each  visit  the  apices  can  each  be  rotated  a  little 
more  up  and  in,  so  as  to  increase  the  amount  of  exercise.  For 
esophoria  the  apices  would  be  down  and  out  in  the  same  way. 
For  hyperphoria,  each  apex  can  be  directed  to  the  right,  but  with 
one  a  little  above  the  horizontal  and  the  other  below  it.  The 
prism  that  has  its  apex  below  the  horizontal  should  be  before  the 
hyperphoric  eye.  When  vertical  and  horizontal  deviations  occur 
together,  it  is  quite  easy  to  train  them  both  simultaneously  by 
adjustment  of  the  prisms. 

All  training  should  be  intermittent  and  be  done  at  set  times, 
when  the  eyes  and  the  system  generally  are  most  free  from 
fatigue. 

Remember,  it  is  conjugate  innervations  that  are  trained  by 
prisms,  and  not  muscles. 

Simple  Rules  for  Heterophoria.— The  following,  though  too 
dogmatic  for  seniors,  are  suggested  for  the  guidance  of  the 
inexperienced  : 

A — Diagnosis. 

(1)  Measure  the  deviation  on  the  5-m.  scale  by  the  disk  of 
red   rods   before  one  eye  and  a   piece  of   green   glass    before  the 
other. 

(2)  Repeat  the  measurement  with  the  head  placed  in  different 
attitudes  to  ensure,  by  proving  concomitancy,  that  no  paralysis  is 
present. 

(3)  Note  the  amplitude  of  convergence,*  or  at  least  the  con- 
vergence near  point. 

(4)  If  the  deviation  be  a  horizontal  one,  test  it  also  in  near 
vision  by  small  tangent  scale  and  square  prism. 

*Kefraction  is  assumed  to  have  already  been  investigated. 


Heterophoria  231 

(5)  If  time  permit,  test  the  breadth  of  fusion,  or  prism  duc- 
tion,  as  it  is  now  called  ;  especially  the  abduction  power  in 
esophoria,  to  see  if  it  reach  its  normal  limit  of  4°,  and  the 
super  and  subduction  in  hyperphoria,  to  confirm  the  reality  of 
the  heterophoria. 

B —  Treatment. 

1 I )  Remember  that  exophoria  and  cyclophoria  in  near  vision 
are  comparatively  unimportant,   and  that  in  distant  vision  hyper- 
phoria is  at  least  four  times  more  worthy  of  notice  for  each  degree 
than  horizontal  deviations. 

(2)  If  deviations  cause  no  symptoms,  leave  them  alone  so  far 
as  special  correction  is  concerned,  yet  do  not  ignore  their  evident 
indications,  if  glasses  are  needed  otherwise,  on  the  choice  of  these 
glasses.     Thus,  hypermetropes  with  esophoria  should  be  fully  cor- 
rected, since  the  relief  of  accommodation  relaxes  the  convergence 
associated  with  it  ;  hypermetropes  with  much  exophoria  should  be 
under-corrected,   and   myopes  with  exophoria  be  fully  corrected  ; 
presbyopes  with  esophoria  in  near  vision  need  stronger  lenses  than 
those  with  exophoria,  and  so  on  (Norton  and  Savage). 

(3)  Deviations  which  cause  some  evident  effort  to  overcome, 
with    feeling   of    strain    and    perhaps   occipital    headache,    require 
treatment.       It   is  generally  best  to   try  simple  correction  of   re- 
fraction first,   decentering  the  lenses,  if  at  all,  only  a  little  in  the 
direction  of  relief.      If  that  fail,  orthoptic  training,  and  if  that  also, 
operation. 

(4)  When  all  these  prove  insufficient,  which  should  rarely  be, 
so  that  heterophoria  needs  relieving  prisms,  I  would  suggest  that  : 

Three-quarters  of  a  persistent  hyperphoria  should  be  corrected 
by  prisms  or  by  decentering. 

Two-thirds  of  a  distant  or  the  whole  of  a  near  esophoria. 

Half  or  a  third  of  a  distant  and  a  quarter  of  a  near  exophoria. 

These  rules,  it  need  scarcely  be  said,  are  only  a  matter  of 
general  judgment,  since  each  case  needs  its  own  study. 

Rules  for  Decentering.— (a)  Mentally  halve  the  deviation,  so 
as  to  divide  its  effect  between  the  two  eyes,  (b*)  Decide  what  pro- 
portion of  each  half  it  is  desirable  to  relieve  (see  last,  section), 
(r)  Remember  that  a  gradient  or  departure  of  i°  is  approximately 
i3/(  in  100  ;  therefore,  since  a  lens  of  i  D.  has  a  focal  length  of 
one  meter,  it  will  have  to  be  decentered  i?4  cm-  to  obtain  i°  of 
deviating  effect  on  light  (see  Fig.  99). 


232  Tests  and  Studies  of  the  Ocular  Muscles 

We  can  at  once  obtain  the  centimeters  of  decentering  required 
for  any  given  number  of  degrees  if  we 

Multiply  the  degrees  by  I  ^ 

and  divide  by  D, 
D,  of  course,  being  the  number  of  diopters  in  the  lens. 

To  put  it  in  a  formula  (if  C  be  the  centimeters  of  decentering 
and  P  the  desired  prismatic  effect)  in  degrees  of  variation, 

C  =     1-\  and/3-:  |  CD. 


Fig.  99 


To  show  the  Prismatic  Effect  of  decentering  ;  each  displacement  of  1%  cm.  producing  -°  devia- 
tion for  each  diopter  in  the  lens.  A  meter  lens  will  have  its  principal  focus  at  1  />,  so  that 
light  falling  on  the  lens  1%  cm.  from  its  optical  center  is  deflected  1°,  another  1%  cm.  2°, 
and  so  on.  A  half-meter  lens  will  have  its  focus  at  2  D.  and  the  corresponding  deflections 
will  be  2°  and  4°.  With  a  5  D  lens  they  will  be  5°  and  10°. 

Prism  Diopters. — For  those  who  use  prism  diopters  the  matter 
is  simpler  still,  since  we  only  divide  the  number  of  prism  diopters 
required  by  the  number  of  diopters  in  the  lens  to  find  the  centi- 
meters of  decentration.  The  formulae  being 

C  =  ^  ;  and  A  =  C  D. 

Direction  of  Decentering.  —Whatever  the  nature  of  the  hetero- 
phoria,  the  following  rules  hold  good  : 

Displace  —  lenses  with  the  deviation. 
Displace  -f-  lenses  against  it. 

For  example,  in  hyperphoria,  before  the  highest  eye 
Decenter  -f-  lenses  downwards. 
Decenter  —  lenses  upwards. 


Heterophoria  233 

The  rationale  is  very  easy  to  understand,  for  every  student 
knows  that  —  lenses  appear  to  displace  objects  with  them,  and  + 
lenses  against  them.  When  an  eye  tends  to  deviate  we  displace 
the  image  in  the  same  direction,  so  as  to  indulge  the  eye  a  little. 

Example.  —  In  a  hypermetrope  of  3D.,  suppose  we  have  de- 
rided to  relieve  a  right  hyperphoria  of  4°  by  i  y2  °  in  each  eye.  Here 
P  =  iy2°  and  D  =  3, 

7  P 

so  that  C  =  --  =-  becomes 
4/? 

X    '*  875 


4X3 

The  right  lens  must  be  displaced  downwards,  therefore,  8^  mm., 
and  the  left  lens  upwards  to  the  same  amount. 

Prism  Diopters.*—  Taking  prism  diopters  —say  we  decide   to 
relieve  each  eye  by  2^  A.     Then 


D  * 

Operative  Interference  is  indicated  in  : 

(1)  Diplopia,    however    occasional,    which    is   too   high   for 
prisms,   provided  not  due  to  any  passing  or  removable   cause  or 
to  progressive  disease  ;  and  if  of  sufficiently  long-standing  to  give 
no  hope  of  natural  cure.     The  correction  of  any  error  of  refraction 
should  be  tried  first. 

(2)  Especially  is  the  homonymous  diplopia  for  distant  objects 
experienced   by  some    myopes  (strabismus  myopicus  convergens) 
suitable  for  operation,  since  diverging  effort  is  greater  than  converg- 
ing ;  yet  we  take  care  not  to  over-correct  if  in  near  vision  there  be 
already  exophoria,  since  the  diminution  of  the  distant  convergence 
is    certain  to  be  accompanied  by  a  corresponding  increase  of  the 
near  divergence.     Advancements  alone  are  suitable. 

(3)  High  esophoria,  especially  if  it  persists  in  near  vision,  is 
generally  suitable  for  operation  if  correction  of  refraction  fails  ;  but 
operation  should  only  be  considered  if  the  suppressed  squint  cause 
subjective  symptoms  or  conscious  strain. 

(4)  Exophoria,  or  latent  divergence,  is  generally  of  very  little 
account  at  all,  and  only  very  rarely  calls  for  operation.     I   have 


*A  gradient,  or  departure,  of  1  in  100  is  a  convenient  unit  and  was  chosen  by  Charles 
F.  Prentice,  M.  E.,  of  New  York,  as  a  unit  of  prismatic  power,  being  called  by  Swan  Bur- 
nett a  w  prism-diopter."  "  Centime  "  would,  perhaps,  be  a  better  name  for  it  when  used  for 
other  purposes  than  prisms.  It  is  a  unit  which  does  not  bear  multiplication,  since  its  higher 
powers  are  not  angular  multiples  of  its  lower. 


234  Tests  and  Studies  of  the  Ocular  Muscles 

seen  some  very  high  degrees,  indeed,  without  suggesting  it.  When 
it  causes  periodic  squint,  or  diplopia,  after  correction  of  refraction, 
the  result  of  operating  is  generally  excellent,  provided  a  sufficient 
change  of  position  is  obtained.  Improvement  of  the  general 
health,  bad  teeth,  or  digestion,  and  errors  of  refraction  need  look- 
ing to  first.  In  America  large  doses  of  tincture  of  nux  vomica  are 
recommended.  Orthoptic  training  is  suitable  for  those  who  will 
take  the  trouble.  The  amplitude  of  convergence  should  always  be 
tested  in  these  cases,  and  if  it  is  good,  operation  is  more  likely  to 
succeed. 

The  case  on  page  148  may  be  taken  as  one  out  of  many  to 
illustrate  the  necessity  for  an  over-effect  at  first.  It  was  that  of  a 
young  school-girl  with  myopia  =  .5  ;  .5,  whose  left  eye  turned 
outwards  when  tried.  The  tangent  strabismometer  showed  the 
deviation  to  be  one  of  —  20°  L  (the  negative  sign  meaning 
divergence,  and  L  the  left  eye).  Under  chloroform,  both  external 
recti  were  divided  and  one  internus  advanced,  producing  con- 
siderable convergence  with  homonymous  diplopia,  which,  when 
measured  a  week  after,  was  -f-  10°  in  the  primary  position. 
Three  weeks  after  the  operation  it  became  O,  and  seven  weeks 
after —  2°,  where  it  seems  to  remain.  The  periodical  "turning 
out"  is  cured  and  the  eyes  are  rapidly  regaining  their  con- 
comitancy  as  measured  by  the  rod  test.  There  is  no  diplopia 
on  looking  in  any  direction. 

(5)  Hyperphoria,  without  occasional  diplopia,  is  rarely  large 
enough  to  be  beyond  the  aid  of  prisms.  When  otherwise,  and  it 
causes  distress,  operation  is  quite  justifiable. 

When  we  want  to  produce  a  small  effect,  we  have  the  choice 
of  the  following  methods  : 

(1)  Stevens  tenotomy. 

(2)  DeWecker's  capsular  advancement. 

(3)  Knapps'  or  Savage's  tendon  shortening. 

(4)  Snellen's  tenotomy  with  a  limiting  suture. 

(5)  Ordinary  advancement  without  tenotomy  of  the  antagonist. 
These  are  described  in  all  the  better  text-books,  so  that  I  need 

only  mention  here  the  convenience  of  using  needles  furnished  with 
Hagedorn  eyes  and  threads  stained  two  or  three  different  colors. 
For  considerable  advancements,  where  three  sutures  are  used,  the 
middle  one  should  be  attached  strictly  in  the  middle  part  of  the 
tendon  and  catch  in  the  sclerotic  close  to  the  cornea,  where  it  is  tied  ; 


Heterophoria  235 

first,  the  side  sutures  being  ready  to  tie  thereupon.  The  more  the 
tendon  is  detached  from  the  capsule  and  from  its  own  conjunctiva, 
the  safer  the  result. 

Graduated  tenotomy  is  another  name  for  careful  tenotomy. 
Partial  "buttonholing"  of  the  middle  of  a  tendon  has  no  effect 
unless  a  moral  one,  since  the  tendons  are  peculiarly  inextensible, 
and  even  a  narrow  strand  at  each  margin  is  enough  to  make  the 
effect  nil.  But  by  dividing  the  tendon  with  great  delicacy,  so  as  to 
leave  the  indirect  attachments  unimpaired,  an  extremely  small 
effect  can  be  produced  ;  and  should  it  not  be  small  enough,  a 
limiting  suture  can  be  employed  as  well.  The  instruments  intro- 
duced by  Stevens  for  this  purpose  are  admirably  adapted  for  it, 
except  that  the  hook  and  forceps  are  just  a  little  too  fine  to  do  the 
best  work,  for  the  same  reason  that  the  finest  catheter  is  not  the 
easiest  for  strictures,  being  more  apt  to  make  a  false  passage. 

Marginal  tenotomy  has  been  much  advocated  by  Savage  with 
a  view  to  correct  at  the  same  time  any  cyclophoria.  After  "  button- 
holing "  the  tendon,  as  in  Stevens'  operation,  one  margin  only  is 
snipped  with  the  scissors.  The  torsion  of  the  eye  is  much  less 
affected  by  this  procedure  in  the  case  of  the  internal  and  external 
recti  than  in  that  of  the  superior  and  inferior,  for  obvious  reasons. 


CHAPTER   XIII 


Cyclophoria 

Let  us  now  consider  this  subject,  at  which  Prof.  Savage  has 
worked  so  much.  Cyclophoria  is  a  tendency  for  the  principal 
meridians  of  the  retina  to  fall  out  of  parallelism  with  each  other 
whenever  the  eyes  are  disassociated,  so  as  no  longer  to  be  engaged 
in  ordinary  binocular  vision. 

By  far  the  commonest  form  is  that  in  which  the  principal 
meridians  diverge  above  (binocular  extorsion).  By  tests  made  in 
near  vision,  Savage  found  it  present  in  at  least  twenty-five  per  cent, 
of  normal  eyes.  Cyclophoria  of  this  kind  has  probably  no  clinical 
importance,  unless  it  is  very  great  or  due  to  a  paresis  of  an  oblique 
muscle.  In  the  latter  case  there  will  be  vertical  diplopia,  either 
latent  or  manifest,  down  and  to  the  opposite  side  from  the  paresed 
superior  oblique.  In  the  absence  of  any  such  tendency  to  vertical 
diplopia  in  the  four  corners  of  the  field,  as  tested  by  the  glass  rod, 
Cyclophoria  must  not  be  attributed  to  the  oblique  muscles,  but  to 
the  innervations.  Should  need  require,  the  slack  innervation  con- 
cerned may  be  strengthened  by  exercise,  but,  in  nearly  all  cases, 
non-paralytic  Cyclophoria  causes  no  symptoms  and  requires  no 
treatment. 

Its  Detection  and  Clinical  Measurement. — When  a  disk  of 
mounted  rods  is  held  before  one  eye  of  a  patient  who  is  engaged 
in  looking  at  a  distant  flame,  it  sometimes  happens  that  while  the 
rods  are  horizontal  the  streak  of  light  created  by  them  appears 
more  or  less  sloping,  and  to  make  it  vertical  the  rods  have  to  be 
tilted  from  the  horizontal. 

Such  a  patient  has  latent  torsion  or  Cyclophoria.  If  the  amount 
be  considerable  and  the  diameters  of  the  disk  be  truly  marked, 
the  degree  of  torsion  can  easily  be  read  off  from  an  astigmatic 
trial  frame. 

Its  Exact  Measurement. — For  very  accurate  work,  the  rods 
would  be  better  mounted,  exactly  horizontal,*  in  a  rigid  stand, 
with  a  long  thin  wand  pivoted  to  the  wall  behind  the  flame,  or  a 


*  Optical  adjustment  is  superior  to  the  use  of  a  spirit  level.    It  can  be  effected  by  adjust- 
ing the  rods  so  as  to  obtain  the  maximum  definition  of  a  thin  vertical  line  on  the  wail. 

(236) 


Cyclophoria  237 

cord  so  fastened  by  one  end  as  to  be  adjustable  to  the  vertical,  or 
to  any  inclination  from  it.  The  tilting  of  the  wand  or  slanting  of 
the  cord  required  to  bring  either  parallel  to  the  streak  of  light, 
represents  the  cyclophoria.  Since,  for  clinical  purposes,  however, 
such  accuracy  is  not  necessary,  simple  rotation  of  the  disk  of  rods 
in  the  trial  frame  till  the  streak  appears  parallel  with  a  fixed  vertical 
or  horizontal  line  on  the  wall  is  quite  sufficient.  Cyclophorias  are 
divided  into  "  paretic  "  and  "  non-paretic. " 

ParetiC  Cyclophoria. — Whenever  leaning  of  an  image  is  ob- 
served to  any  marked  extent,  the  possibility  of  slight  paresis 
of  one  of  the  obliques  should  not  be  overlooked,  and  measures 
should  be  taken  accordingly  to  discover  whether  the  tilting 
varies  on  looking  in  certain  directions  ;  also  whether,  on  looking 
up  and  down,  any  inconcomitant  hyperphoria  can  be  demonstrated 
by  the  rods. 

Non-paretic  Cyclophoria. — Tilting  which  is  not  paretic  is  due 
to  slackness  of  one  of  the  conjugate  innervations.  It  will  be 
remembered  that  there  are  three  or  four  of  such  innervations  con- 
nected with  torsion — one  causing  parallel  dextrotorsion,  another 
parallel  laevotorsion,  another  conjugate  intorsion,  and  yet  another 
(perhaps)  conjugate  extorsion. 

Explanation  Of  Leaning  Image. — It  may  be  well  to  explain  the 
relation  between  the  tilting  of  the  streak  of  light  and  the  torsion  of 
the  eye. 

When  a  flame  is  fixed  with  both  eyes  without  any  apparatus,  a 
vertical  line  inscribed  on  the  wall,  passing  through  the  flame, 
throws  its  image  on  the  vertical  meridian  of  each  retina,  these  verti- 
cal meridians  being  kept  parallel  to  each  other  by  torsional  innerva- 
tion,  in  order  to  combine  the  two  pictures  into  one. 

As  soon,  however,  as  a  glass  rod  is  placed  before  one  eye,  all 
active  innervation  exerted  in  the  interest  of  single  vision  ceases  and 
the  eye  rolls  into  its  position  of  dissociation. 

While  the  glass  rod  is  horizontal  the  picture  formed  by  it 
upon  the  retina  remains  geometrically  vertical  and,  therefore,  now, 
as  soon  as  torsion  occurs,  this  linear  picture  falls  on  a  new  meridian 
of  the  retina  which  is  not  the  originally-vertical  one,  and  is  pro- 
jected according  to  the  rule  that  the  false  image  is  displaced  in  the 
opposite  direction  to  the  displacement  of  the  eye.  A  dextrotorted 
streak,  therefore,  means  a  laevotorted  eye,  and  vice  versa. 

As  soon  as  the  rod  is  tilted  so  as  to  make  the  retinal  picture 


238  Tests  and  Studies  of  the  Ocular  Muscles 

fall  on  the  originally-vertical  meridian  of  the  eye,  the  streak  appears 
vertical.* 

Rule  for  Rod  Test. — This  very  simple  rule,  therefore,  may  be 
made  that  the  torsion   of  the  rod,   required   to   make  the  streak 
appear  vertical,  represents  exactly  the  torsion  of  the  eye,  both  in 
sense  and  amount. 

Cyclophoria  in  Near  Vision. — Dr.  Savage,  of  Nashville,  has 
worked   much  at  the  subject  of  latent  torsion  in  near  vision  by  a 

test    of  his  own,   for  which  he 

^ — ^— — — ^^— ^^^—      utilized   the    author's     double 

prism  of  Fig.  91. 

On    looking    at    a     card 
— — — — — — ^— — - — — ^— ^^—      marked  with  a  horizontal  line, 

F!g-  10°  through  the  double  prism  held 

Dext retorsion  of  the  naked  eye,  causing  appa-      t     r    _  4.U  *• 

rent  laevotorsion  of  the  middle  linear  image         Delore     One      eye,       the     patient 

sees  two  parallel  false  images 

of  the  line  and  a  third  real  image  (Fig.  100)  between  them,  which 
slopes  with  respect  to  the  other  two  in  a  sense  opposite  to  the 
torsion  of  the  eye  that  sees  it. 

Dr.  Savage  attempted  to  cure  it  by  exercising  the  eyes  in  the 
opposite  direction,  using  a  weak  cylindrical  lens  for  the  purpose, 
rotation  of  which  tilts  the  image  of  a  vertical  object  seen  through 
it.  For  exercises  in  distant  vision,  Duane's  suggestion  is  a  good 
one — to  use  two  disks  of  glass  rods,  one  for  each  eye,  and  gradually 
rotate  them  in  opposite  directions  while  endeavoring  to  keep  the 
streak  of  light  from  doubling  ;  thus,  with  the  bi-prism  before  the 
left  eye,  the  appearance  is  generally  as  in  Fig.  100,  showing  excyclo- 
phoria.  This  condition  is  so  common  as  to  deserve  being  regarded 
as  physiological.  For  near  vision,  a  steroscope  such  as  suggested 
by  Perry,  or  Javal's  "stereoscope  a  cinque  mouvements,"  could 
be  used  ;  or,  perhaps  best  of  all,  Helmholtz's  rotating  prisms, 
which  enable  an  object  to  appear  gradually  rotated  ;  but  the  author 
does  not  think  that  cyclophoria  which  appears  in  near  vision  only, 
needs  treatment  at  all. 

Oblique  Astigmatism. — When  an  eye  with  oblique  astigmatism 
looks  at  a  vertical  line,  the  image  of  that  line  on  the  retina  is 
twisted  from  the  vertical  towards  the  meridian  of  maximum  corneal 
curvature.  The  image  thrown  from  a  horizontal  line  is  likewise 

*  For  physiological  experiments  I  have  devised  a  more  delicate  test,  in  which  any  ten- 
dency to  fusion  is  more  entirely  abrogated  (Ophthalmic  Review,  June,  1894),  but  not  be'ing  a 
clinical  test,  it  has  no  place  here. 


Cyclophoria 


239 


twisted,  also  towards  the  same  corneal  meridian.  To  verify  these 
facts  let  the  reader  look  at  a  cross  line,  as  in  a,  Fig.  100^,  with 
the  right  eye,  after  having  placed  before  it  a  strong  minus  cylinder 
with  the  axis  down  and  in.  The  appearance  is  as  shown  in  6, 
Fig.  ioo^,  each  arm  of  the  cross  being  twisted  toward  the  axis  of 
the  cylinder.  A  similar  cylinder  with  its  axis  down  and  in  before 
the  left  eye,  the  right  being  shut,  would  give  the  appearance  of 
<r,  Fig.  ioo>^. 

It    is    evident  that  were  no  rotation  of  the  globes  about  the 
lines  of  fixation  permissible,  the  effect  with  both  cylinders  together 


a  e  t 

riff.  100% 

would  be  as  in  d,  Fig.  100^.  What  really  occurs,  however,  is  that 
both  eyes  become  either  extorted  or  intorted,  according  as  we  con- 
fine our  attention  to  the  vertical  line  or  the  horizontal  one. 

At  e,  Fig.  100^,  is  shown  the  appearance  when  the  vertical 
line  is  attracting  most  attention,  its  double  images  being  fused  in 
consequence  of  binocular  extorsion  of  the  eyes,  which  allows  each 
image  to  fall  on  the  principal  meridian  of  its  retina  ;  by  this  very 
act  the  angular  separation  betwen  the  horizontal  images  becomes 
doubled.  In  /",  Fig.  100^,  the  horizontal  line  has  attracted  atten- 
tion and  its  double  images  have  been  brought  together  by  binocular 
intorsion,  which  has  doubled  the  angular  separation  between  the 
vertical  images. 


240  Tests  and  Studies  of  the  Ocular  Muscles 

This  constant  alternation  of  the  adjustment  of  the  eyes  about 
their  fixation  lines  is,  no  doubt,  what  accounts  for  the  greater  fre- 
quency of  headaches  in  oblique  astigmatism  as  compared  with  other 
kinds,  nor  can  it  be  corrected  in  any  other  way  than  by  the  cor- 
rection of  the  astigmatism. 

Too  exclusive  attention  to  the  horizontal  images,  together  with 
the  supposition  the  whole  picture  of  an  object  is  tilted  on  the 
retina  in  the  same  direction  as  its  horizontal  lines,  has  led  some  to 
suppose  that  the  correction  of  certain  kinds  of  astigmatism  throws 
strain  on  the  superior  obliques  and  that  of  other  kinds  on  the 
inferior  obliques.  But  the  above  simple  treatment  of  the  subject 
shows  that  the  correction  of  oblique  astigmatism  relieves  both  pairs 
of  obliques  ;  or,  to  put  it  more  correctly,  gives  less  work  to  the 
two  innervations  which  govern  binocular  extorsion  and  intorsion. 

Since  Savage  called  attention  to  the  effect  of  the  correction  of 
oblique  astigmatism,  the  subject  has  attracted  attention.  If  an 
oblique  cylinder  be  held  before  a  normal  eye,  a  square  figure 
looks  drawn  out  or  shrunken  in  a  direction  perpendicular  to  the 
axis  of  the  cylinder,  according  as  the  latter  is  convex  or  concave, 
so  as  to  illustrate  the  fact  that  both  vertical  and  horizontal  lines 
are  tilted,  against  the  axis  of  a  convex,  and  with  the  axis  of  a  con- 
cave cylinder,  when  its  axis  is  oblique.  Were  this  all>  every  side 
of  the  square  would  appear  double  or,  at  least,  indistinct ;  but  the 
mind  prefers  to  see  two  sides  clearly,  even  though  at  the  expense 
of  the  other  two,  and  this  desired  end  is  attained  by  either  conju- 
gate intorsion  or  extorsion  of  the  eyes,  according  to  taste.  Hori- 
zontal lines  in  near  vision  are  preferred  generally  to  be  seen  dis- 
tinctly at  the  expense  of  the  vertical.  In  an  astigmatic  eye  this  state 
of  matters  is  permanent,  and  the  corrective  torsion  is  a  life-habit. 
When,  therefore,  oblique  cylinders  are  prescribed,  this  life-habit 
has  no  longer  any  raison  d'  etre,  and  ceases.  Whether  in 
selecting  the  best  axis  for  the  cylinder,  we  do  well  to  take  account, 
as  Savage  suggests,  of  the  altered  torsional  conditions,  is  very 
questionable. 

It  should  be  remembered  that  latent  torsion  is  much  more 
common  in  near  vision  than  in  distant  vision,  and  when  it  exists  in 
both,  is  apt  to  be  the  greater,  being  in  fact  to  a  certain  extent 
physiological,  and,  I  believe,  analogous  in  its  own  spheres  to  the 
exophoria  so  generally  found  in  near  vision,  in  the  domain  of  the 
converging  innervation. 


Cyclophoria  241 

Cydophorometers.— Several  instruments  of  this  name  have 
been  invented  for  the  measurement  of  cyclophoria  at  a  distance 
rather  than  in  near  vision.  The  first  to  be  published  was  that  of 
Price,  who  placed  mounted  glass  rods  vertically  before  both  eyes, 
with,  in  addition,  a  double  prism,  ridge  horizontal,  before  one  eye.' 
Similar  instruments  followed,  with  some  improvement  of  detail  by 
Baxter,  and  Brewer,  and  others  ;  some  with  single  prism  and 
others  with  double,  and  all  excellently  planned.  When  a  point  of 
light  is  looked  at  through  these  instruments  one  eye  sees  its  hori- 
zontal line  of  light  inclined  with  respect  to  the  line  or  lines  seen  by 


Fig.  101 

Optomyomeler  of  the  Geneva  Optical  Company 

the  other  eye  when  cyclophoria  is  present,  and  the  measurement  is 
effected  by  rotating  one  disk  in  a  graduated  arc  till  the  lines  are 
all  parallel. 

Two  more  forms  of  apparatus  for  latent  torsion  deserve 
description. 

Oplomyometer.— The  first  is  the  "  optomyometer "  of  the 
Geneva  Optical  Company,  shown  in  Fig.  101.  It  consists  essen- 
tially of  two  tubes,  nearly  twenty  inches  long,  one  of  which  is 
capable  of  horizontal  movement  only,  while  the  other  can  be 
elevated  or  depressed  to  any  required  angle.  The  patient  is  made 
to  look,  with  both  eyes,  down  these  tubes,  and  sees  a  thin  slit  cut 
in  a  rotating  disk  at  the  far  end  of  each.  By  a  little  adjustment  of 
the  movable  tube  its  own  slit  can  be  made  to  appear  vertically 
above  the  other,  and  then  if  one  slants  with  reference  to  the  other, 
the  disk  is  rotated  till  the  slant  is  corrected.  If  the  patient  have 
any  latent  torsion,  it  will  lie  found  that  when  the  slits  appear  to 


242  Tests  and  Studies  of  the  Ocular  Muscles 

him  to  have  the  same  direction,  they  actually  are  inclined  to  or 
from  each  other  to  an  extent  which  exactly  measures  his  latent 
deviation.  In  a  variation  of  the  experiment,  one  slit  can  be  made 


Fig.  102 
Stevens'  Clinoscope 

to  appear  to  the  patient  to  lie  across  the  other  one  at  right  angles 
to  it  ;  when  if  a  latent  torsional  deviation  be  present,  the  slits  will 
be  found  to  be  really  inclined  to  each  other  by  a  greater  or  less 
angle  than  90°. 

Clinoscope. — Dr.  Stevens  has  improved  on  this  instrument,  in  his 
clino^cope,  which  consists  of  two  tubes  nearly  twenty  inches  long, 
mounted  on  a  brass  platform.  The  attachment  to  the  platform  per- 
mits the  tubes  to  be  adjusted  in  parallelism,  in  convergence  or  in 
divergence,  and  the  platform  itself  is  attached  by  a  movable  joint 
to  an  upright  standard,  so  that  the  tubes  can  be  given  any  desired 
dip  simultaneously.  The  tubes  can  be  rotated  about  their  longitu- 
dinal axes  by  thumb-screws,  and  this  motion  is  recorded  by  an 
index-pointer  above  each  tube.  At  the  far  end  of  each  tube  pro- 
vision is  made  for  maintaining  diagrams  in  position.  An  example 
of  these  figures  is  shown  in  Fig.  103,  which  represents  two  pins, 
one  to  be  seen  by  each  eye.*  The  heads  of  the  pins  blend,  and 
by  rotating  one  till  the  pins  form  a  continuous  straight  line,  the 
latent  torsion  is  measured. 


'Vo'kmann's  Apparatus. — A  design  similar  to  this  appears,  according  to  the  language 
of  Helniholtz,  to  have  been  that  employed  by  Volkmann  :  "  Instead  of  a  whole  diameter  on 
his  rotarv  disks,  he  only  traced  one  radius,  and  endeavored  on  binocular  examination  to  make 
these  radii  appear  in  the  same  straight  line.  The  head  was  suitably  held:  the  rotary  disks 
were  placed  in  two  darkened  tubes  which  could  be  directed  at  will  by  the  aid  of  s'uitable 
joints,  so  that  each  eye  should  see  one  disk  through  each  of  the  tubes,  the  disk  remaining 
always  perpendicular" to  the  line  of  fixation." 


CyclopJioria 


243 


To  measure  the  amplitude  of  torsion,  similar  disks  are  used, 
but  with  a  complete  diameter,  instead  of  a  radius  (Fig.  104), 
on  each.  These  diameters  are  fused,  and  by  rotating  both  in  oppo- 
site directions  till  they  begin  to  separate  the  strength  of  the  faculty 
of  fusion  is  measured. 

Dr.  Stevens  finds  the  amplitude  of  extorsion  for  each  eye  to 
be  11°  and  that  of  intorsion  to  be  slightly  less.  This  holds  good 
when  both  eyes  are  simultaneously  extorted  or  simultaneously 
intorted. 

He  finds,  however,  that  he  cannot,  while  maintaining  the 
vertical  position  of  one  of  the  lines  in  the  clinoscope,  rotate  the 
other  to  an  extent  double  of  that  to  which  the  two  were  rotated. 
With  one  line  vertical  he  can  only  slant  the  other  to  right  or  left  by 
about  14°,  without  breaking  fusion. 

Curiously  enough,  he  says  that  horizontal  lines  are  not  held 
in  fusion  nearly  so  easily  as  vertical  ones,  the  amplitude  for  each 
eye  being  only  3°  inwards  and  3°  outwards. 


He  says  that  during  artificial  binocular  extorsion  or  intorsion 
the  united  line  appears  concave  or  convex,  according  to  the  will  of 
the  observer. 

Meissner'S  Test,  1858.— It  is  only  for  convenience  that  Meiss- 
ner's  test  is  included  in  this  chapter,  since  the  phenomena  of 
torsion  manifested  by  it  are  not  strictly  those  of  "latent"  torsion, 


244 


Tests  and  Studies  of  the  Ocular  Muscles 


but  of  physiological  actual  torsion  during  single  binocular  vision  for 
near  objects.  This  is  apt  to  be  confused  with  latent  torsion,  to 
which,  indeed,  it  is  closely  related. 

It  appears  that  in  ordinary  binocular  vision  of  near  objects  both 
eyes  rotate  outwards  about  their  optic  axes  (binocular  extorsion), 
and  the  more  so  the  nearer  the  object  becomes. 

This  species  of  torsion  increases  when  the  visual  plane  is 
elevated  and  lessens  as  it  is  depressed,  till  it  at  last  disappears 


•••I 

hi 


Fig.  1O5 

copic  Figures  slightly  inclined  (LeConte) 


altogether,  when  the  fixation  lines  are  depressed  45°  below  the 
horizontal  ;  if  depressed  more  than  that,  intorsion  of  both  eyes  is 
apt  to  show  itself. 

Meissner  proved  these  points  by  taking  a  metallic  thread, 
stretching  it  perpendicularly  to  the  plane  of  fixaticTn  and  looking  at 
it  in  such  a  way  as  to  make  the  visual  lines  converge  to  a  point 
situated  a  little  beyond  or  a  little  behind  this  thread.  He  found 
the  double  images  of  the  thread  not  parallel  but  relatively  intorted, 
showing  that  the  eyes  are,  to  the  same  degree,  extorted.  By 
moving  the  lower  end  of  the  thread  nearer  the  observer  and  the 
top  farther  away,  so  as  to  introduce  an  element  of  perspective  (the 
top  of  the  thread  being  now  farther  from  the  eyes  than  the  bottom) 
the  double  images  can  be  made  parallel.  The  amount  of  the  pre- 
vious intorsion  of  the  images  can  easily  be  calculated  from  the 
amount  of  inclination  required  to  be  given  to  the  thread  to  bring 
the  double  images  to  parallelism. 

Le  Conte  has  carefully  confirmed  Meissner' s  results,  differing 
in  one  point  only,  namely,  that  while  the  latter  believed  that  a 
greater  inclination  must  be  given  to  the  thread  as  vision  becomes 
nearer,  Le  Conte  finds,  with  his  own  eyes  at  least,  an  inclination  of 
7°  or  8°  to  be  that  required  for  all  near  distances. 


Cydophoria  245 

Depression  Of  the  Visual  Plane.—  Meissner  found  that  the  more 
the  visual  plane  and  the  thread  were  simultaneously  depressed  (the 
mid-point  of  the  thread  being  kept  at  a  uniform  distance  from  the 
eyes  all  the  time),  the  less  the  thread  had  to  be  displaced  from  the 
perpendicular  to  the  visual  plane,  till,  with  a  depression  of  45°  it 
needed  no  displacement  at  all.  In  this  position  of  the  eyes,  there- 
fore, the  torsion  we  are  considering  becomes  nil.  Helmholtz  calls 
this  the  "primary  position  of  the  eyes  for  convergence,"  defining 
primary  position  as  that  of  zero  torsion  (Nullpunkt  der  Raddrehun- 
gen),  and  stating  that  in  convergence  the  eyes  have  a  lower  pri- 
mary position  than  when  the  visual  axes  are  parallel.  In  his  own 
case  he  found  zero  lie  one  day  a  little  higher  and  another  day  a 
little  lower,  and  even  to  become  altered  during  a  series  of  experi- 
ments. It  is  useless,  therefore,  to  attempt  too  great  a  precision  in 
denoting  it. 

True  Primary  Position  in  Distant  and  in  Near  Vision.—  In  dis- 
tant vision,  Helmholtz  defined  the  primary  position  for  the  parallel 
motions  of  the  eyes  as  that  in  departing  from  which,  in  any  cardinal 
direction,  no  false  torsion  was  generated.  During  convergence  he 
also  tested  the  deviations  from  Listing's  law  in  the  secondary  posi- 
tions of  the  eyes,  and  found  them  to  be  such  as  to  confirm  the  idea 
of  the  primary  position  in  convergence  being  one  of  depressed 
visual  axes. 

Le  Conte'S  Confirmation.  —  Le  Conte's  experiments  showed  that, 
with  the  point  of  fixation  at  the  following  distances  from  the  root  of 
the  nose  the  torsion  was  shown  in  this  little  table  : 

At  7  inches  —  each  eye  became  extorted  i^° 


n    22"  "       "         "  "          5 

t<  !/     «  ^       ««         «  "        10° 

74 
On  looking  up,  i.  e.,   with  elevation  of  the  visual  plane,  the 

extorsion  increases,  which  Le  Conte  attributes,  no  doubt  truly,  to 
the  inferior  obliques  ;  on  looking  down,  as  already  described,  it 
becomes  less. 

Savage's  Test  Compared.—  All  this  shows  that  in  making 
Prof.  Savage's  test  with  the  double  prism,  note  should  be  taken 
both  of  the  distance  at  which  the  test  is  made  and  of  the  inclina- 
tion of  the  head,  for  though  his  test  is  not  the  same  as  Meissner's, 
since  the  two  eyes  are  thoroughly  dissociated  (whereas,  in  Meiss- 
ner's they  are  not  dissociated  at  all),  there  is  no  doubt  the  results 


246  Tests  and  Studies  of  the  Ocular  Muscles 

include  the  phenomena  here  treated  of.  His  test  is  doubtless  a 
good  one  in  its  place,  but  its  value  should  be  carefully  differentiated. 
For  example,  when  torsional  defects  are  apparent  in  distant  vision 
to  any  marked  degree,  it  is  of  service  to  also  investigate  the  con- 
ditions in  near  vision  to  see  if  they  present  any  great  departure 
from  what  Meissner  showed  to  be  physiological. 

As  already  confessed  that  in  the  clinical  study  of  latent  torsion, 
I  do  not  attach  much  significance  to  near  vision  phenomena,  unless 
coupled  with  distant  vision  defects  ;  but  it  is  perhaps  well  not  to 
overlook  the  possibility  of  rare  cases  in  which  Savage's  and  Meiss- 
ner's  tests  might  show  great  anomalies. 

Eaton's  Apparatus. — Perhaps  the  best  rough  clinical  way  to 
institute  Meissner' s  test  is  that  suggested  by  Eaton  of  a  strip  of 


*.  toe 

Eaton's  mode  of  makiug  Meissner's  Test 

wood  grooved  at  one  end  to  rest  on  the  root  of  the  nose,  with  a 
white  metal  plate  rigidly  fixed  to  the  other  extremity  so  as  to  hang 
down  therefrom  at  such  an  angle  as  to  be  perpendicular  to  the 
visual  plane,  when  a  black  dot  in  its  middle  is  fixed  by  the  patient. 
A  long  hat  pin,  with  its  head  downwards,  is  stuck  by  its  point  into 
the  under  part  of  the  strip  of  wood  an  inch  or  two  from  the  end 
and  parallel  to  the  metal  plate.*  This  is  shown  in  Fig.  106. 

To  use  the  apparatus  :  First  depress  the  whole  till  the  images 
of  the  hat  pin  become  parallel  ;  this  discovers  the  amount  of  depres- 
sion which  must  be  imparted  to  the  visual  axes  to  obtain  zero 
torsion.  Secondly,  by  making  the  patient  look  straight  forward, 

*  A  graduated  arc  might  with  advantage  be  arranged  to  indicate  the  inclination  of  the 
pin,  as  shown  in  the  figure. 


Cychphoria  247 

the  hat  pin  can  be  slanted  so  as  to  bring  its  head  nearer  the  patient 
till  parallelism  of  the  double  images  is  obtained — the  amount  of  slant 
showing  the  amount  of  extorsion.  Lastly,  by  elevating  the  appara- 
tus the  increase  of  the  torsion  on  looking  up  can  be  demonstrated. 

Much  more  elaborate  and  accurate  apparatus  could  be  devised, 
but  for  clinical  purposes  they  might  induce  us  to  make  much  of 
little.  I  have  made  the  following  simple  rule,  which  affords  a 
sufficiently  close  approximation  for  clinical  work :  Multiply  the 
number  of  degrees  by  which  Meissner's  thread  has  to  be  inclined  to 
make  the  images  parallel,  by  half  the  interocular  distance  in  centi- 
meters (generally  about  3.2)  and  divide  by  the  distance  of  the 
center  of  the  thread  in  centimeters  ;  this  gives  the  torsion  of  each 
eye  in  degrees. 

Formula  for  Meissner's  Test.  — The  formula  I  obtained,  where 
/  is  the  inclination  of  the  thread  in  degrees,  T  the  torsion  of  each 
eye  and  C  the  angle  of  convergence  for  each  eye,  is  : 

Tan.  T  =  Tan.  /  Sin.   C. 

Putting  arcs  as  equivalent  to  tangents,  the  formula  becomes  : 

T=  I  Sin.  C. 

From  which  we  see  :  ( i )  That  for  any  fixed  distance  of  the 
thread  the  torsion  of  the  eye  increases  proportionately  to  the  incli- 
nation of  the  thread,  and  (2)  that  for  any  given  and  constant 
inclination  of  the  thread  the  torsion  varies  with  the  sine  of  the 
convergence. 

Thus,  with  the  thread  50  cm.  (about  20  inches)  away,  the 
torsion  of  each  eye  would  be  .064  of  the  inclination  of  the  thread  ; 
at  25  cm.  (about  10  inches)  it  would  be  twice  as  much,  namely, 
.128  ;  at  20  cm.  (about  8  inches)  .16  ;  at  15  cm.  (about  6  inches) 
.21  ;  at  10  cm.  (about  4  inches)  .32  ;  and  so  on. 

It  only  remains  to  show  how  to  arrive  at  the  formula. 

How  Formula  Obtained. — Let  us,  for  convenience,  call  that  meridian  of 
each  retina  which  is  vertical  while  both  eyes  look  straightforwards  at  distant 
objects,  the  originally  vertical  meridian,  and  that  plane  which  passes  through 
it,  as  well  as  through  the  point  of  fixation,  the  originally  vertical  plane. 

During  distant  straightforward  vision  the  originally  vertical  planes  of  the 
two  eyes  are  parallel,  since  both  are  vertical,  and  they  intersect  each  other 
in  a  vertical  line  passing  through  the  point  of  fixation. 

Were  the  eyes  to  experience  no  torsion  during  the  act  of  convergence, 
this  line  of  common  section  would  still  remain  vertical  for  every  distance  of 
fixation.  But  any  torsion  of  the  eyes  is,  of  course,  accompanied  by  equal 


248  Tests  and  Studies  of  the  Ocular  Muscles 

rotatations  of  their  originally  vertical  planes  about  their  axes  of  fixation,  and 
though  the  originally  vertical  planes  must  still  intersect  one  another  in  a 
straight  line  passing  through  the  point  of  fixation,  that  line  remains  no 
longer  vertical,  but  has  its  upper  end  inclined  towards  the  observer,  if  the 
case  be  one  of  intorsion,  and  away  from  the  observer  if  the  case  be  one  of 
extorsion.* 


Fig.  107  Fig.lOS 

Author's  plan  of  solving  Meissner-Torsion 

The  greater  the  rotation  of  the  originally  vertical  planes,  during  any 
constant  distance  of  fixation,  the  greater  is  the  tilt  backwards  or  forwards 
of  their  line  of  common  section. 

On  the  other  hand,  to  produce  a  constant  tilt  of  the  line,  greater  torsion 
is  needed  for  every  increase  of  convergence. 

Now,  it  is  only  when  Meissner's  thread  is  held  parallel  to  this  line  of 
common  section  of  the  originally  vertical  planes,  that  its  images  appear 
parallel  to  one  another.  Given  the  distance  of  fixation,  we  learn  at  once 
from  the  inclination  which  we  have  to  give  to  Meissner's  thread  what  is  the 
rotation  of  the  originally  vertical  planes,  and  thus  the  torsion  of  the  eyes. 

Fig.  108  gives  a  horizontal  plan  of  the  problem,  E  P  being  the  left  axis 
of  fixation,  meeting  the  median  plane  (M P)  at  the  point  of  fixation  P,  so 
that  C  is  the  angle  of  convergence  for  the  left  eye. 

The  left  originally  vertical  plane  passes  through  the  axis  of  fixation  E  P 
and  rotates  about  it  in  strict  association  with  the  torsion  of  the  eyeball,  thus 
intersecting  the  median  plane  in  a  straight  line,  which  ever  passes  through 
P,  either  perpendicularly  to  the  plane  of  the  paper  (as  when  no  torsion 
exists)  or  with  more  or  less  inclination  from  the  perpendicular  (according 
to  the  torsion). 

Select  in  the  originally  vertical  plane  an  imaginary  line  running  parallel 
to  the  axis  of  fixation,  and  at  any  unit-distance  from  it.  So  long  as  the 
originally  vertical  plane  is  vertical  this  line  will  lie  immediately  over  the  axis 
of  fixation  and,  in  the  horizontal  plan  of  our  figure,  appear  to  coincide  with 
it.  If  the  eye  be  extorted,  however,  the  line  will  occupy  some  such  position 
*  The  reader  to  whom  this  is  not  self-evident,  may  think  of  the  prow  of  a  canoe. 


Cyclophoria  249 

as  that  shown  in  plan  by  S Sf,  meeting  the  median  plane  at  5  (which  we 
may  regard  as  the  upper  extremity  of  Meissner's  thread,  when  the  middle 
of  the  thread  is  fixed  by  the  eyes  at  P}. 

Now,  designating  the  angle  of  torsion  by  7",  the  ordinary  rule  of  hori- 
zontal projection  gives  us 

R  P  =  Sin.  T, 

and  it  is  evident,  from  the  figures,  that 

„  R  P    _  Sin.  T         ,  . 

*      *J     r-.  •  s~,  —    pr-          7^.  (  I  ) 

Sin.  C       Sin.  C 

But  P  S  equals  the  horizontal  co-ordinate  of  the  upper  half  of  Meiss- 
ner's thread,  shown  in  side  elevation  in  Fig.  107,  as/  S.     Whence — 
Since  Pp  was,  by  construction,  taken  as  unity, 

P  S  =  Tan.  t. 
Therefore,  from  (i) 

„         .         Tan.   T 

Tan.  t  =  -=-. — . 

Sin.  C 

and 

Tan.  T  =  Tan.  i    Sin.  C 

Which  means  approximately  that  the  torsion  of  each  eye  is  directly  propor- 
tional to  the  inclination  of  the  thread,  and  also  to  the  amount  of  convergence 
in  true  meter  angles.  True  meter  angles  are  found  by  measuring  the  dis- 
tance of  the  point  of  fixation  from  the  center  of  the  eye,  and  finding  how 
many  times  that  distance  will  go  into  a  meter. 

Most  readers  will  agree  that  this  subject  is  a  difficult  one. 


CHAPTER   XIV 


The  Eye  in  Darkness 

The  problem  now  before  us  *  is  to  discover  how  an  eye  behaves 
when  it  is  covered  by  the  hand,  or  otherwise  placed  in  total  dark- 
ness, while  its  fellow  is  still  actively  engaged  in  near  vision. 

We  have  already  seen  that,  during  deep  sleep,  the  eyes  gen- 
erally experience  exotropia,  as  proved  by  simple  inspection.  But 
when  awake  in  darkness  it  is  clear  that,  except  for  more  pronounced 
deviations  than  those  which  occur  physiologically,  we  cannot  solve 
this  difficult  problem  by  direct  inspection  of  the  eye  or  through 
Javal's  ground  glass  ;  neither  can  after-images  afford  us  any  assis- 
tance, since  movements  resulting  from  alterations  of  the  conver- 
gence innervation  are  just  those  which  after-images  do  not  betray. 

Even,  therefore,  if  we  were  to  gaze  steadily  at  a  source  of  light 
till  it  became  impressed  on  the  retina  before  darkening  the  eye  with 
the  hand,  the  eye  might  then  move  under  the  hand  without  any 
apparent  movement  of  the  after-image. 

What  is  needed  is  an  apparatus  capable  of  placing  an  eye  sub- 
jectively in  absolute  darkness,  and  yet  able  to  take  account  of  its 
movements.  To  solve  this  problem  by  utilizing  the  blind  spot  I 
devised  the  visual  camera  in  1882.  It  need  hardly  be  recalled  that 
the  blind  spot  (or  "punctum  caecum,"  discovered  by  Mariotte)  is 
an  approximately  circular  gap  in  the  field  of  vision  of  each  eye, 
which  was  shown  by  Donders  to  be  due  to  the  fact  that  the  entire 
surface  of  the  optic  disk  is  wholly  insensible  to  light.  There  is  an 
area,  therefore,  in  the  field  of  vision  of  each  eye  which  is  entirely 
devoid  of  visual  impressions  and  large  enough,  according  to 
Helmholtz,  for  eleven  full  moons  to  stand  in  a  row  in  it.  The 
center  of  the  blind  area  lies  about  15°  to  the  outside  of  the  point 
of  fixation,  and  its  diameter  subtends  an  angle  of  about  6°. 

The  "camera"  consists  of  a  light  wooden  box,  represented  in 
Fig.  109,  blackened  inside,  and  of  a  somewhat  wedge-like  or 
pyramidal  shape,  its  dimensions  being  about  a  foot  from  side  to 
side  and  about  nine  inches  from  before  backwards.  It  is  one  inch 
deep  along  the  curved  border  and  inclines  gradually  to  the  depth  of 

*  Trans.  Oph.  Soc.,  1882-3,  and  Jour,  of  Anat.  and  Phys.,  vols.  xx  and  xxi. 

(250) 


The  Eye  in  Darkness  251 

half  an  inch   at  the  narrow  end.      The  latter  is  provided  with  two 
visual  apertures  pierced  through  slides  a  a  which  permit  of  mutual 
approximation  or  the  reverse,  and  between  them  is  a  groove  for  the 
nose.      A   fixed  median   parti- 
tion  b  extends  to  within    two 
inches   of    the    middle  of   the 
curved  border  and  is  crossed 
by   a    small    transverse    ' '  ob- 
structive"  c,   which  is  merely 
a  little  piece  of  wood  suspen- 
ded through  a  slit  in  the  roof, 
in  which  it  can  slide  from  side 
to  side.     The    curved  end    of 
the  box  is  built  up  of  two  arcs 
d  d,  each  described  about  the  Flg-  109 

.  .  ...  The  Visual  t'amer* 

center  of  motion  of  its  corres- 
ponding eye  and  mited  by  a  straight  piece  64  mm.  long,  in 
the  center  of  which  is  a  tiny  fixed  aperture  e  covered  by  thin 
paper,  bearing  a  printed  letter  so  as  to  ensure  accurate  accom- 
modation. On  either  side  of  this  arc  are  two  movable  aperturesyy, 
preferably  colored  red  and  green  respectively,  and  pierced  through 
brass  slides  ^  s,  which  travel  so  that  each  luminous  point  can 
be  moved  at  pleasure  along  its  own  half  of  the  curved  end  inde- 
pendently of  the  other.  This  is  made  possible  by  a  system  of 
long  slits,  so  cut  in  the  brasswork  that  the  luminous  points  can 
be  made  to  travel  without  admitting  any  adventitious  light,  since 
the  slits  mutually  overlap  each  other.  The  brass  slides  are  marked 
in  degrees,  which  indicate  the  angular  distance  of  each  luminous 
point  from  the  central  aperture. 

To  make  our  first  experiment,  adjust  the  two  lateral  apertures 
ff  so  that  each  shall  be  15°  distant  from  the  central  aperture  e, 
and  place  the  obstructive  in  the  middle  of  its  path  as  represented 
by  the  shading  c.  Now  look  into  the  camera,  holding  it  so  as  to 
let  the  light  fall  on  the  three  small  apertures  fe  f  but  for  which 
its  interior  is  quite  dark,  and  gaze  at  the  printed  letter  in  the 
central  aperture  e  ;  while  doing  so  the  two  lateral  apertures  ff  will 
be  invisible,  since  they  fall  on  the  blind  spot  of  each  eye,  and  are 
lost  to  view.  The  circles  of  shading  which  are  represented  in  the 
figure  surrounding  the  apertures  //  correspond  to  the  two  blind 
spots  and  are  at  that  distance  about  an  inch  in  diameter. 


252  Tests  and  Studies  of  the  Ocular  Muscles 

Now  suddenly  push  the  obstructive  to  the  right,  into  the  posi- 
tion shown  by  g  in  the  figure.  The  mind  remains  quite  unconscious 
that  the  vision  of  the  fixation  aperture  e  by  the  right  eye  is  cut  off 
by  this  action,  though  the  aperture  appears  a  little  dimmer.  Now, 
however,  the  right  eye  is  subjectively  in  absolute  darkness,  since 
the  left  luminous  point  f  is  hidden  by  the  median  partition  b,  the 
central  fixation  aperture  e  is  hidden  by  the  obstructive  g  and  the 
right  lumiuous  point  /is  lost  in  the  blind  area.* 

The  fixation  aperture  <?,  however,  begins  at  once  to  appear 
moving  slowly  towards  the  right,  and  in  a  few  moments  the  lumi- 
nous point  f  springs  into  "view,  showing  that  the  eye  has  been 
moving  sufficiently  for  its  blind  area  to  no  longer  contain  the  lumi- 
nous aperture,  and  the  two  visible  apertures  appear  nearer  to  each 
other  than  they  really  are.  If  we  attempt  to  touch  each  of  them 
separately,  by  a  finger  outside  the  box,  the  average  of  a  number  of 
attempts  would  show  that  the  right-hand  aperture  f  appears  dis- 
placed a  little  to  the  left  of  its  true  position,  and  the  fixation 
aperture  appears  displaced  a  little  to  the  right  of  its  true 
position. 

Now  withdraw  the  right-hand  luminous  point  outwards,  till  it  is 
again  lost  to  view  in  the  blind  area  ;  it  will  remain  lost,  showing 
that  the  eye,  having  moved  outwards  a  certain  extent,  remains 
there.  It  is  easy  to  localize  the  inner  border  of  the  blind  area 
before  and  after  the  push  given  to  the  obstructive,  and  thus  to 
measure  in  degrees  the  amount  of  deviation  suffered  by  the  eye. 
The  amount  varies  in  different  persons  ;  in  some  it  is  nil.  but  on 
the  average  nearly  4°  of  outward  deviation  takes  place. 

In  the  following  table,  which  collects  the  results  obtained  from 
ten  different  persons,  the  average  amount  of  deviation  is  a  little 
too  high  to  be  representative.  It  shows,  however,  how  greatly  the 
amount  of  deviation  varies  in  different  individuals.  The  first  two 
columns  show  what  is  the  angular  interval  between  the  inner  and 
outer  borders,  respectively,  of  the  blind  area  and  the  visual  axis, 
the  difference  between  which  gives,  of  course,  the  angular 
dimensions  of  the  blind  area. 

In  my  own  case  the  deviation  varies  from  3°  to  7°  or  even  8°. 
according  to  the  time  of  day,  the  state  of  health,  the  previous 
occupation  of  the  eyes  and,  I  think,  also  the  temporary  compara- 

*In  fact,  the  only  illumination  of  the  right  eye  is  gathered  into  a  tiny  image  tJ0  of  an 
inch  wide  in  the  center  of  the  optic  disk. 


The  Eye  in  Darkness 


253 


tive  anaemia  or  congestion  of  the  brain.  It  appears  to  be  greater 
in  the  morning  than  in  the  evening,  and  less  after  much  reading, 
especially  when  the  head  is  congested  from  close  application  or 
hot  rooms. 

TABLE  I 


No. 

Inner  border  of 
blind  area 

Outer  border  of      Breadth  of 
blind  aera           blind  area 

DKVIATION 

I 

I2^° 

I8X° 

5^° 

0° 

2 

12/2° 

iS'/2° 

6° 

i°  or  y2° 

3 

I  2  5*2° 

19° 

6^° 

2^° 

4 

11° 

17° 

6° 

4° 

5 

12^° 

18^° 

6° 

4K° 

6 

12^° 

i8#° 

6° 

5° 

7 

13° 

19° 

6° 

6^° 

8 

13° 

i8K° 

5K° 

7° 

9 

iiK° 

17° 

5^° 

7° 

10 

18^° 

6° 

7K° 

Averag 

e  

4*° 

The  physiological  experiments  which  can  be  made  with  this 
camera  are  too  numerous  to  be  all  treated  here. 

Sense  Of  Projection. — Fig.  no  shows  one  in  which  two  tiny 
apertures,  each  seen  by  only  one  eye,  when  separated  a  certain 
distance,  appear  superimposed.  This  distance  measures  the  exo- 
phoria  for  the  distance  of  the  base  of  the  camera,  and  by  asking 
the  patient  to  touch  the  superimposed  apertures,  his  sense  of 
projection  can  be  tested. 

Effect  of  Attention  on  the  Desire  for  Fusion. — An  instructive 
experiment  consists  in  making  the  two  eyes  see  one  image 
double,  while  a  third  image  is  seen  by  one  eye  only.  If  attention 
is  fixed  upon  this  third  image,  though  there  is  nothing  to  pre- 
vent the  diplopia  from  being  overcome,  there  is  found  to  be  no 
desire  to  overcome  it,  even  though,  by  the  nature  of  the  experi- 
ment, the  double  images  are  perfectly  focused,  being  in  the  same 
plane  as  the  third  one  on  which  the  attention  is  fixed.  But  the 
moment  attention  is  paid  to  either  of  the  double  images,  they 


254 


Tests  and  Studies  of  the  Ocular  Muscles 


instantly  run  together.  This  experiment  throws  much  light  on 
the  relation  between  attention  and  the  desire  for  single  vision. 

To  make  the  necessary  arrangement  of  the  camera,  the  right 
lateral  aperture  is  placed  two  degrees  to  the  right  of  the  central 
one,  the  former  being  filled  in  or  covered  with  a  piece  of  paper 
marked  with  a  printed  letter,  and  the  latter  covered  with  tissue 
paper,  and  therefore  blank.  On  looking  into  the  camera,  with  the 

"stop"  in  the  middle,  the  two 
points  are  seen  in  their  true  posi- 
tion, the  printed  letter  appearing  to 
the  right  of  the  blank  aperture. 
Now  push  the  ' '  stop  ' '  to  the  right ; 
the  two  images  begin  to  move 
slowly  together  till  superimposed, 
and  continue  this  movement  till 
they  have  just  changed  places.  The 
lettered  aperture  is  now  about  2° 
to  the  left  of  the  blank  one.  While 
attentively  examining  the  letter,  re- 
place the  "stop"  in  the  middle; 
immediately  another  blank  image 
appears  to  the  left  of  the  lettered 
one,  so  that  the  letter  has  now  one 
blank  image  on  each  side  of  it  and 
at  equal  distance  from  it,  and  this 

arrangement  remains  so  long  as  the  letter  receives  chief  attention, 
but  the  moment  either  blank  image  is  looked  at,  it  and  its  fellow 
run  together,  the  letter  returning  to  its  true  and  original  projec- 
tion to  the  right.  This  illustrates  many  points  in  the  laws  of 
conjugation. 

Speed  Of  the  Exophoria. — The  rate  of  relaxation  of  conver- 
gence can  be  measured  by  timing,  from  a  pendulum,  the  move- 
ments of  a  "stop."  It  begins  with  me  in  less  than  half  a  second 
and  goes  on  with  decreasing  speed,  which  becomes  inappreciable 
after  from  half  a  minute  to  a  minute  and  a  half. 

Laws  Of  Conjugation  Illustrated. — Several  more  experiments 
can  be  made  to  illustrate  these  on  the  principles  mentioned  in 
Chapter  V,  showing  that  the  oculo-motor  muscular  sense  is  purely 
central,  the  same  contraction  of  a  muscle  being  mentally  appre- 
ciated in  one  way  or  another  according  entirely  to  the  innervation 


Fig.  110 

The  Camera  used  to  test  Projection 


The  Eye  in  Darkness  255 

in  play.  Such  phenomena  as  the  following  are  very  evident  with 
my  own  eyes. 

When  one  eye  is  excluded  the  object  fixed  by  the  seeing  eye 
appears  to  move  in  the  same  direction  as  the  deviation  of  the 
covered  eye,  but  at  half  the  rate  and  through  half  the  angle. 

Thus  a  fixed  object,  watched  by  a  stationary  eye  may  appear 
to  move,  though  in  another  experiment  the  same  fixed  object  seen 
by  a  moving  eye  may  appear  stationary.  It  all  depends  on  what 
conjugate  innervations  are  in  play. 

An  image  on  the  fovea,  whatever  be  the  real  position  of  the 
object  it  belongs  to,  appears  in  my  case  to  be  referred  to  the  plane 
which  bisects  the  angle  of  convergence,  and  which  therefore 
passes  through  the  point  of  fixation  and  a  point  midway  between, 
and  slightly  behind,  the  centers  of  the  two  eyes.  This  plane  is 
shown  by  y  p'  in  Fig.  no  and  c  b  in  Fig.  33. 

In  spite  of  exophoria,  the  nervous  connection  between  con- 
vergence and  accommodation  is  most  sensitive,  since  the  slightest 
increase  of  the  latter  is  accompanied  by  an  approximation  of  the 
double  images. 

False  Fusion. — When  two  apertures  are  arranged  for  each  to 
be  seen  by  only  one  eye,  the  desire  to  unite  them  increases  in  pro- 
portion to  their  apparent  nearness,  and  may  be  greater  than  the 
desire  to  unite  true  double  images,  if  the  separation  of  the  latter 
from  each  other  be  greater.  By  making  their  heights  different,  the 
images  tend  to  keep  as  near  as  they  can  to  the  same  vertical  line, 
even  if  they  cannot  overcome  the  vertical  separation  ;  but  this 
tendency  ceases  if  the  difference  in  height  be  more  than  2°  or  3°. 

Diluted  Fusion. — By  moving  the  "stop"  from  the  center  of 
its  path  to  one  or  other  end  of  its  path,  at  measured  brief  intervals, 
the  vision  of  a  single  aperture  can  be  made  alternately  monocular 
and  binocular.  The  monocular  intervals  favor  exophoria,  while 
the  binocular  intervals  tend  to  prevent  it.  By  regulating  the 
intervals,  exophoria  may  at  pleasure  either  be  prevented  or  allowed 
to  go  on  at  any  blower  rate  than  normal,  or  be  arrested  and  main- 
tained at  any  part  of  its  course,  or  be  made  to  slowly  decrease. 
The  desire  for  single  vision  is  lessened,  or  diluted,  as  it  were,  by 
monocular  intervals. 


INDEX 


Abduction 56 

Accommodation 91 

Centers  for ...    92 

Line  of  equal  92 

Relative  image  of % 

Adduction 56 

Aerial  perspective  ...  lln 

After-images 37 

Altitude,  motion  in 48 

Amblyopia,  congenital .  120 

Congenital  without  squint  ....  123 

Ex-anopsia 120 

Tobacco 201 

Amblyoscope,  Worth's 147 

Angle  alpha  of  Donder's 138 

Landolt's 138 

Angle  gamma    .  138 

In  astigmatism     202 

In  cataract  and  iridectoruy     .   .    .  203 

Vnsyminetrical 203 

Anisometropia,  fallacy  from 137 

Anoephoria 101 

Anterior  pole 16 

Arc  Keratoscopique,  De  Wecker's    .   .      141 

Masselon's      141 

Arc  of  contact      56 

Astigmatism,  oblique 238 

Axes,  tilted 66 

Model  with 76 

Axis  of  the  eye 16 

Agreed        50 

Azimuth,  motion  in       48 


Ball  of  the  eye  16 

Binocular  depression     81 

Dext  reduction      81 

Dextrotorsion 82 

Elevation 81 

Extorsion 83 

Fixation 109,  205 

Intorsion 83 

Laevoduction 81 

Laevotorsion      82 

Parallel  movements 43 

Vision      109 

Blind  spot 250 

G 

Cardinal  motions 36 

Caruncle         29 

Center  of  motion  of  eyeball 55 

Centrifugal  impulses 79 


Cerebellum,  lesion  of 84 

Charpentier's  method 140 

Chart,  How  to  read  a  multiple 166 

How  10  read  a  simple     .   .       .   .      165 

Charts,  How  to  transfer 168 

Chorea .    81 

Cliuoscope,  Steven.-' .242 

Collarette 21 

Collimation  of  the  visual  line 200 

Concoiuitancy 204 

Test  for       .       225 

Measurement  of 143 

Convergence ;»:; 

And  accommodation 88,    91 

Angle  of 93,    96 

Horizontal      192 

Line  of  equal 92 

Miners' 193 

Mixed 198 

Near  point  of 196 

Protean       193 

Rotation  of 192 

.Searching 193 

Test  for 190 

Unilateral 193 

Varieties  of 192 

Vertical 198 

Conjugate  depression-paralysis     ....  187 

Deviations  and  paralysis 187 

Elevation-paralysis 187 

Lateral  deviation 188 

Conjugation  of  the  two  eyes 79 

Rules  of      89 

Co-ordination 73 

Cornea,  Paths  of  the 76 

Corneal  nebulae    .   .          120 

Images,  ophlhalmoscopic     ....  198 

Corneal  orbit     72 

Plane  of 72 

Reflections,  ophthalmoscopic  .   .      137 
Reflection,  fixation  ]>osition  of  the,  200 

Corresponding  points 103 

Cyclophoria 236 

Detection  of 236 

In  near  vision 238 

Measurement  of 236 

Non-paretic 

Paretic 237 


D 

Darkness,  The  eye  in 250 

Deorsumductors 167 


257 


Tests  and  Studies  of  the  Ocular  Muscles 


Dextroduction 49,    56 

Dextroductors 167 

Dextrotorsion 42 

Motion  of 45 

Deviation  secondary 135 

Consecutive 149 

Direction  of      214 

Latent      97 

Measurement  of 143 

Reason  of 136 

Deviometer,  Worth's .  .  145 

De  Wecker's  capsular  advancement          234 

Diplopia 104,  157 

Breadth  of 108 

Crossed 108 

Distal 104 

Homonymous 108 

Monocular      158 

Peripheral 96 

Physiological 104 

Post-operative 125 

Proximal 104 

Suppressed     122 

Direction  of  objects 91 

Displacement,  horizontal 74 

Dissociation  of  the  eye 213 

Graefe's  device  for 213 

Distance  of  objects     91 

Donder's  Law 36,    40 

Plane  of  reference 41 

Dynamics  of  the  eye 70 

Dynamometer,  Landolt's 154 


Eaton's  apparatus 216 

Elevation  angle  of  fixation 49 

Equator  of  the  eye 16 

Error  of  approximation 207 

Esophoria 129,215 

Exophoria 75,    96,  215 

In  near,  oblique,  vision 95 

Physiological 227 

Speed  of 254 

Exophthalmos 17 

Paralytic 75 

Extorsion 56 


Faculties,    extension  of  partially    pre- 
served   133 

Recovery  of  lost 131 

Field  of  projection 103 

Fixation 44,  101 

Axis  of 100 

Binocular    ....          101 

Central 100 

Central  imperfect 123 

Defective  diagnosis  of 124 


Fixation 

Eccentric 124 

Field  of 124 

Line  of 44,  100 

Lost 123 

Persistent 99 

Point  of 44,    98 

Reflex 99 

Fovea  centralis 200 

Anatomical 100 

Foveal  differentiations 99 

Foveal  projections 103 

Fusion 106 

Breadth  of 108,  227 

Diluted 255 

Faculty,  deficiency 121 

False 265 

Horizontal  breadth     ....     109,  229 

Negative      109 

Positive 109 

Sense,  training  the 147 

Total  amplitude  of 109 

Tubes 131 

Vertical 109 

Q 

Giddiness 157 

Globe  of  the  eye 15 

Translations  of  the 30 

Graefe's  view 79 

H 

Helmholtz,  analysis  of  ocular  motions  .    49 

Law  ......          40 

Plane  of  reference 41 

Hering's  drop  test 114 

Heterophoria 97,  108,  215 

Correction  of 228 

Decentering  rules  for 231 

Diagnosis  of 230 

Direction  of      232' 

IB  near  vision 226 

Physiological 214 

Simple  rules  for 230 

Treatment  of 231 

Hirschberg's  method     .   .   . 139 

Hooke's  law  .  . 22 

Hyperophoria,  congenital 84 

Hysteria 81 


Image,  leaning,  suppression  of  the  false  .  122 

Tilting  of 39 

Trembling  of  the 124 

True      107 

Images,  suppression  of 104 

Impulse 94 

And  work  .  .    95 


Index 


259 


Initial  position  of  fixation  plane  ....  49 

Inuervatious .81 

Conjugate 81,  86 

Convergence 81,    87,  91 

Dextrodueting 86 

Horizontal  conjugate     86 

Laevoducting 86 

Parallel 90 

Variations      81 

Voluntary 81 

Intorsion     .   . 56 

Isogonai  line 88 

K 

Kataphoria 101 

Kinetic  energy  37 

Kuapp's  tendon  shortening 234 


Laevoduction    ..........    49,    56 

Laevoductors     .............  137 

Laevotorsion  ..........    42 

Landolt's  figure  for  field  of  fixation    .  .  101 
I>e  Conte's  experiment  .........    38 

lecture  coutrollee   ...........  113 

Levator  palpebne     ......    .....    29 

Ligaments,  check     ...........    20 

Extensibility     ..........    23 

External  .....    ........    27 

External  superior    ........ 

Inferior    .............    31 

Inferior  oblique   ........    " 

In  teuotomy      ..........    26 

Internal  ............    28 

Lines  of  force    .............    58 

Listing's  Law    .............    3e 

Proof  of  ............    35 

Reasons  for    ...........    37 

Listing's  plane  ............    40 

Vertical  diameter  of  .......    46 

Love  of  single  vision     .........  106 


Maddoxrod  ..............  220 

Manufacture  of    ........  220 

Mode  of  use  .....       .....  221 

Test  for    ..........     220,226 

Mai-projection  .......    502,  106,  156 

Mathematical  perspective    .......  110 

Mauthner's  hypothesis  ........    75 

Meissner's  test  .............  243 

Formula  for      .   .       .......  247 

Meissner's  torsion,  plan  of  solving  .  .   .  248 
Meridians  of  the  eye     .........    16 

Mnemonics     ...........     161,  185 

Mobility,  comparative  .........  I53 

Conjugate   ............  153 


Motions,  normal  ...........    17 

Testing  horizontal  ........  160 

Motorfield  ......   .........  101 

Homony  mous  restriction  of  ...  187 

Medial  depression   ...........    58 

Elevation  ............  58 

Muscular  axes  ............   58 

Functions  ............  53 

Muscular  anomalies,  photography  of  .  .  208 
Coue  ...............    19 

Planes  ..............    60 

Planes,  properties  of  .......  60 

Muscles    ......   .....  17,   52,   68 

Associate        ...........    68 

Cardinal  groups  of  ........  160 

Medial  origins  of     ........    57 

Oblique    ............   53 

Recti     ..............    52 

Single  spasm  of  .........  80 

Motion,  monocular  ...........    87 

Movements  of  the  eye  .......   15,    48 

Of  the  parallel  ..........  37 

N 

Nystagmus        .............  192 

Curious  case  of    .........  196 

Etiology  of    ...........  194 

Examination  of   .........  194 

Excursions  in    ......   ...   .195 

Motions    .............  193 

Rotation      ......       .....    82 

Treatment  of     .........     196 


o 

Ocular  movements  ........... 

Object  of  ........  .... 

Precision  of  the       ........ 

Silence  of  the       ....... 

Swiftness  of  the    ......... 

Ocular  motions     ............ 

Laws  of  ........... 

Ocular  muscles         .  ........ 

Secondary  effect  of  ........ 

Occlusion     .    .  ......          •   • 

Ophthalmotropes     ........... 

Anderson  Stuart's  ........ 

Landolt's    ........   .... 

Optic  foramen   ............ 

Optomyometer      ............ 

Orbit  of  the  eye  ............ 

Axisot     ............. 

Capacity  of    .......... 

Outlet  of  ............ 

Shape  of  ............. 

Orthophoria      ........... 

Orthoptic  training  ........     137, 


63 
66 
63 
18 
241 
18 
18 


230 


Parallax 

Crossed 


260 


Tests  and  Studies  of  the  Ocular  Muscles 


Parallax 

Homonymous 216 

Left 216 

Right 216 

Test  of  Duane 216 

Paralysis 160 

Isolated 74 

Left  external  rectus       170 

Left  inferior  oblique 179 

Left  inferior  rectus    ....  176 

Left  internal  rectus 172 

Left  superior  oblique 177 

Left  superior  rectus       174 

Of  third  nerve 180 

Operative  treatment  in 80 

Right  external  rectus 171 

Right  inferior  oblique 178 

Right  inferior  rectus     175 

Right  superior  rectus 173 

Right  superior  oblique 176 

Single   . 61 

Paralysis  conjugate 88 

Precise  tests  for 189 

Rough  tests  for         188 

Paralysis  ocular 151 

Examination  of 153 

Measurement  of 182 

Symplons  of 151 

Paralytic  equilibrium 149 

Semi-orbits 77 

Pathetic  nerve .   .  181 

Percival's  experiment 89 

Phorometer 217 

Prince's 217 

Risley's 217 

Stevens'       218 

Perception  of  distance  . 102 

Binocular 110 

Monocular ...  109 

Of  relief Ill 

Pole,  anterior .   .    16 

Posterior 16 

Primary  position 35,    39 

Priestley  Smith's  tape  method 144 

Prism  diopters 232 

In  near  vision          227 

Perimeter  method 139 

Perimeter  method,  Scluveigger's  .  155 

Prism  test    ...  216 

Deviating  angle  of 107 

Prism,  deorsumducent         228 

Sursumducent 228 

Prisms,  power  of  overcoming 107 

Prismatic  displacement,  apparent    ...  107 

Projection 102 

Field  of 102 

Illustrative  errors  of 105 

Line  of  direction 102 

Origin  of  .    .  105 

True  test  for  .  .  .  105 


Pupillary  light-reflex 121 

Pupillary  reflection 32 

Pulley  of  superior  oblique 54 


Recorded  reflections 212 

Recti  muscles 52 

Description  of 52 

Insertion  of 53 

Relative  strength  of 53 

Refraction  surmisable 202 

Rente's  experiment 37 

Retinal  horizon 41 

Retractor  muscle 17 

Rod-test,  rule  for 238 

Rotation  34 

Amount  of 70 

Axis  of 60,    70 

Center  of  34 

Composition  of 69 

Paralytic     78 

Resolution  of 7.S 

Sense  of  .  .70 


s 

Savage's  tendon,  shortening 23 

Test 245 

Schurrman's  figures 101 

Semiluuar  membrane 29 

Sphenoidal  fissure          IS 

Snellen's  test,  Worth's  modification  of  .  145 

Squint      133 

Apparent 137 

Camera 20!) 

Convergent,  over  correction  of  .  .  128 
Convergent,  under  correction  of  .  138 
Divergent,  over  correction  of  ...  128 
Divergent,  under  correction  of  .  .  128 

Evidences  of 13;i 

Exclusion  test  for .   .  133 

Fixed  convergent 130 

Infantile 146 

Latent      134 

Manifest 134 

Squinting  eyes,  deficient  adduction  sec- 
tion of  the  right  eye 72 

Spasm  lateral 881 

Idiopathic 80 

Steadiness  of  the  eyeball 15 

Stereoscope,  Berry's   .' 113 

Magnetic 132 

Wheatstone's 131 

Stereoscopic  bar  reader  and  squint      .   .  129 

Stereoscopic  vision 109 

Stereoscopic  experiments .  112 

Screen  test,  subjective 135 

Strabismometer,  Lawrence's 138 


Index 


261 


Strabismometry,  Priestley  Smith's  mode 

of 206 

Linear      103 

Screen  test,  subjective 216 

Screen  test,  objective 216 

Subjective 148 

Strabismus 115 

Accommodative 118 

After  treatment  for 128 

Alternating 115 

Anisometropia      115 

Comitant 115 

Convergens  concomitans .   .       .      116 

Couvergeus  myopicus 125 

Convergent    without  hypermetro- 

pia 119 

Definition  of 115 

Deviation  of      115 

Divergent 127 

Horizontal 115 

Incomitaut     115 

In  myopia      127 

Non-paralytic 115 

*aralytic 115 

Periodic 214 

Unilateral 115 

Vertical 115 


Tangent  scales 223 

Advantages  of 224 

Hirschberg's  224 

Landolt's 224 

Tenon's  capsule 18,  149 

Fascia 18 


Tenon's 

space 18 

Tenotoiny    .    .  80 

Double  prismatic 229 

For  convergent  squint  ..'....  147 

Graduated      235 

Marginal 235 

Stevens' 234 

M'cllen's 234 

i'rial  frame 228 

Torsion     ....  35 

Calculator 42 

False 42 

Geometry  of 46 

Index  of 42 

Paretic     46 

Secondary       36 

Touching  point 56 

Trcipnmeter.  Stevens'     190 

Tunica  ad veiititia  ocua    .  .  25 


Vasa  uorticosa is 

Vision  direct 98 

Indirect 98 

Visual  camera  .  .  16,  251 

As  a  test  tor  oiojection 254 

Visual  piane 41 

w 

White  spot 89 

Worth's  amblyoscope     M7 

Four-dot  test ...  145 

Tubes    .  ......  137 


Clinics 

In 

Optometry 


CLINICS  IN  OPTOMETRY 

BY  c.  H.  BROWN,  M.  D. 

Graduate  University  of  Pennsylvania;  Professor  of  Optics  and  Refraction;  formerly  Physician 

in  Philadelphia  Hospital;  Member  of  Philadelphia  County,  Pennsylvania 

State  and  American  Medical  Societies 

"Clinics  in  Optometry"  is  a 
unique  work  in  the  field  of  practical 
refraction  and  fills  a  want  that  has 
been  seriously  felt  both  by  oculists 
and  optometrists. 

The  book  is  a  compilation  of 
optometric  clinics,  each  clinic  being 
complete  in  itself.     Together  they 
cover  all  manner  of  refractive  eye 
defects,  from  the  simplest  to  the 
most  complicated,  givingin  minutest 
detail  the  proper  procedure  to  fol- 
low in  the  diagnosis,  treatment  and  correction  of  all  such  defects. 
No  case  can  come  before  you  that  you  cannot  find  a  similar 
case  thoroughly  explained  in  all  its  phases  in  this  useful  volume, 
making  mistakes  or  oversights  impossible  and  assuring  correct 
and  successful  treatment. 

The  author's  experience  in  teaching  the  science  of  refraction 
to  thousands  of  pupils  peculiarly  equipped  him  for  compiling 
these  clinics,  all  of  which  are  actual  cases  of  refractive  error  that 
came  before  him  in  his  practice  as  an  oculist. 

A  copious  index  makes  reference  to  any  particular  case,  test 
or  method,  the  work  of  a  moment. 

Sent  postpaid  on  receipt  of  $2.50 


Published  by 

THE  KEYSTONE  PUBLISHING  CO. 
PHILADELPHIA,  U.  S.  A. 


THE   OPTICIAN'S    MANUAL 

VOL.  I. 


By  C.  H.  Brown,  M.  D. 

Graduate    University    of    Pennsylvania:    Professor    of    Optics    and    Refraction;    formerly 

Physician   in   Philadelphia    Hospital;    Member   of   Philadelphia    County, 

Pennsylvania    State    and    American    Medical    Societies. 


The  Optician's  Manual,  Vol.  I,  was 
the  most  popular  and  useful  work  on 
practical  refraction  ever  written,  and  has 
been  the  entire  optical  education  of 
many  hundred  successful  refractionists. 
The  knowledge  it  contains  was  more  ef- 
fective in  building  up  the  optical  profes- 
sion than  any  other  educational  factor. 
It  is,  in  fact,  the  foundation  structure  of 
all  optical  knowledge  as  the  titles  of  its 
ten  chapters  show: 

Chapter        I. — Introductory  Remarks. 

Chapter       II. — The  Eye  Anatomically. 

Chapter     III.— The  Eye  Optically;  or,  The  Physiology  of  Vision. 

Chapter      IV.— Optics. 

Chapter      V. — Lenses. 

Chapter     VI. — Numbering  of  Lenses. 

Chapter  VII. — The  Use  and  Value  of  Glasses. 

Chapter  VIII.— Outfit  Required. 

Chapter     IX. — Method  of  Examination. 

Chapter       X. — Presbyopia. 

The  Optician's  Manual,  Vol.  I,  was  the  first  important 
treatise  published  on  eye  refraction  and  spectacle  fitting.  It 
is  the  recognized  standard  text-book  on  practical  refraction, 
being  used  as  such  in  all  schools  of  Optics.  A  study  of  it  is 
essential  to  an  intelligent  appreciation  of  its  companion  treatise, 
The  Optician's  Manual,  Vol.  II,  described  on  the  opposite 
page.  A  comprehensive  index  adds  much  to  its  usefulness  to 
both  student  and  practitioner. 

Bound  in  Cloth — 422  pages — colored  plates  and  illustrations. 
Sent  postpaid  on  receipt  of  $2.50 


Published  by 

THE  KEYSTONE  PUBLISHING  CO. 
PHILADELPHIA,  U.  S.  A. 


THE  OPTICIAN'S  MANUAL 

VOL.    II. 


By  C.  H.  Brown,  M.  D. 

Graduate    University    of    Pennsylvania;    Professor    of    Optics    and    Refraction;    formerly 

Physician   in   Philadelphia   Hospital;   Member  of   Philadelphia  County, 

Pennsylvania   State   and    American    Medical    Societies. 


OPTICIAN5 
'  MANUAL 


The  Optician's  Manual,  Vol.  II.,  is 
a  direct  continuation  of  The  Optician's 
Manual,  Vol.  I.,  being  a  much  more 
advanced  and  comprehensive  treatise. 
It  covers  in  minutest  detail  the  four 
great  subdivisions  of  practical  eye  re- 
fraction, viz: 

Myopia. 
Hypermetropia. 
Astigmatism. 
Muscular  Anomalies. 


It  contains  the  most  authoritative  and  complete  re- 
searches up  to  date  on  these  subjects,  treated  by  the  master 
hand  of  an  eminent  oculist  and  optical  teacher.  It  is  thor- 
oughly practical,  explicit  in  statement  and  accurate  as  to  fact. 
All  refractive  errors  and  complications  are  clearly  explained, 
and  the  methods  of  correction  thoroughly  elucidated. 

This  book  fills  the  last  great  want  in  higher  refractive 
optics,  and  the  knowledge  contained  in  it  marks  the  standard 
of  professionalism. 

Bound  in  Cloth — 408  pages — with  illustrations. 
Sent  postpaid  on  receipt  of  $2.50 


Published  by 

THE  KEYSTONE  PUBLISHING  CO. 
PHILADELPHIA,  U.  S.  A. 


THE 

PRINCIPLES  OF  REFRACTION 

in  the  Human  Eye,  Based  on  the  Laws  of 
Conjugate  Foci 

BY  SWAN   M.  BURNETT,  M.  D.,  PH.  D. 

Professor  of  Ophthalmology  and  Otology  in  the  Georgetown  University  Medical  School ; 

Director  of  the  Eye  and  Ear  Clinic,  Central   Dispensary  and  Emergency 

Hospital  ;  Ophthalmologist  to  the  Children's  Hospital  and  to 

Providence  Hospital,  etc.,  Washington,  L>.  C. 


In  this  treatise  the  student  is  given  a  condensed  but  thor- 
ough grounding  in  the  principles  of  refraction  according  to  a 
method  which  is  both  easy  and  fundamental.  The  few  laws 
governing  the  conjugate  foci  lie  at  the  basis  of  whatever  pertains 
to  the  relations  of  the  object  and  its  image. 

To  bring  all  the  phenomena  manifest  in  the  refraction  of  the 
human  eye  consecutively  under  a  common  explanation  by  these 
simple  laws  is,  we  believe,  here  undertaken  for  the  first  time. 
The  comprehension  of  much  which  has  hitherto  seemed  difficult 
to  the  average  student  has  thus  been  rendered  much  easier.  This 
is  especially  true  of  the  theory  of  Skiascopy,  which  is  here  eluci- 
dated in  a  manner  much  more  simple  and  direct  than  by  any 
method  hitherto  offered. 

The  authorship  is  sufficient  assurance  of  the  thoroughness 
of  the  work.  Dr.  Burnett  is  recognized  as  one  of  the  greatest 
authorities  on  eye  refraction,  and  this  treatise  may  be  described 
as  the  crystallization  of  his  life-work  in  this  field. 

The  text  is  elucidated  by  24  original  diagrams,  which  were 
executed  by  Chas.  F.  Prentice,  M.  E. ,  whose  pre-eminence  in 
mathematical  optics  is  recognized  by  all  ophthalmologists. 

Bound  in  Silk  Cloth. 

Sent  postpaid  to  any  part  of  the  world  on  receipt  of  price, 

$1.50 


Published  by 

THE  KEYSTONE  PUBLISHING  CO. 
PHILADELPHIA,  U.  S.  A. 


PHYSIOLOGIC  OPTICS 

Ocular  Dioptrics — Functions  of  the  Retina — Ocular 
Movements  and  Binocular  Vision 


By  Dr.  M.  Tscherning 

Director  of  the  Laboratory  of  Ophthalmology  at  the  Sorbonne,  Paris 


AUTHORIZED  TRANSLATION 

By  Carl  Weiland,  M.  D. 

Former  Chief  of  Clinic  in  ths  Eye  Department  of  the  Jefferson  College  Hospital. 
Philadelphia,  Pa. 


This  book  is  recognized  in  the  scientific  and  medical 
world  as  the  one  complete  and  authoritative  treatise  on 
physiologic  optics.  Its  distinguished  author  is  admittedly 
the  greatest  authority  on  this  subject,  and  his  book  embodies 
not  only  his  own  researches,  but  those  of  the  several  hundred 
investigators  who,  in  the  past  hundred  years,  made  the  eye 
their  specialty  and  life  study. 

Tscherning  has  sifted  the  gold  of  all  optical  research  from 
the  dross,  and  his  book,  as  now  published  in  English,  with 
many  additions,  is  the  most  valuable  mine  of  reliable  optical 
knowledge  within  reach  of  ophthalmologists.  It  contains  380 
pages  and  212  illustrations,  and  its  reference  list  comprises  the 
entire  galaxy  of  scientists  who  have  made  the  century  famous 
in  the  world  of  optics. 

The  chapters  on  Ophthalmometry,  Ophthalmoscopy,  Ac- 
commodation, Astigmatism,  Aberration  and  Entoptic  Phenom- 
ena, etc. — in  fact,  the  entire  book  contains  so  much  that  is  new, 
practical  and  necessary  that  no  refractionist  can  afford  to  be 
without  it. 

Bound  in  Cloth.     380  Pages,  212  Illustrations. 
Price  $3.00 


Published  by 

THE  KEYSTONE  PUBLISHING  CO. 
PHILADELPHIA,  U.  S.  A. 


OPHTHALMIC  LENSES 

Dioptric  Formulae  for  Combined  Cylindrical  Lenses, 

The  Prism-Dioptry  and  Other  Original  Papers 


By  Charles  F.  Prentice,  M.E. 


A  new  and  revised  edition  of  all  the  original  papers  of  this  noted 
author,  combined  in  one  volume.  In  this  revised  form,  with  the  ad- 
dition of  recent  research,  these  standard  papers  are  of  increased  value. 
Combined  in  one  volume,  they  are  the  greatest  compilation  on  the 
subject  of  lenses  extant. 

This  book  of  over  200  pages  contains  the  following  papers: 

Ophthalmic   Lenses. 

Dioptric  Formula?  for  Combined  Cylindrical  Lenses. 

The   Prism-Dioptry. 

A  Metric  System  of  Numbering  and  Measuring;  Prisms. 
The  Relation  of  the  Prism-Dioptry  to  the  Meter  Angle. 
The  Relation  of  the  Prism-Dioptry  to  the  Lens-Dioptry. 

The  Perfected  Prismometer. 

The  Prismometric   Scale. 

On    the    Practical    Execution    of   Ophthalmic   Prescriptions    in- 
volving" Prisms. 

A  Problem  in  Cemented  Bi-Focal  Lenses,  Solved  by  the  Prism- 
Dioptry. 

Why    Strong    Contra-Generic   Lenses   of   Equal    Power   Fail    to 
Neutralize  Each  Other. 

The  Advantages    of   the    Sphero-Toric    Lens. 

The   Iris,  as   Diaphragm   and  Photostat. 

The  Typoscope. 

The  Correction  of  Depleted  Dynamic  Refraction   (Presbyopia). 

PRESS  NOTICES  ON  THE  ORIGINAL  EDITION: 
Ophthalmic    Lenses. 


"The  work  stands  alone,  in  its  present 
form,  a  compendium  of  the  various  laws 
of  physics  relative  to  this  subject  that  are 
so  difficult  of  access  in  scattered  treat- 
ises."— Neie  England  Medical  Gazette. 

"It  is  the  most  complete  and  best  il- 
lustrated book  on  this  special  subject  ever 
published." — Horological  Review,  New 
York. 


"Of  all  the  simple  treatises  on  the 
properties  of  lenses  that  we  have  seen, 
this  is  incomparably  the  best.  .  .  . 
The  teacher  of  the  average  medical  stu- 
dent will  hail  this  little  work  as  a  great 
boon."— Archives  of  Ophthalmology,  ed- 
itnl  by  H.  Knapp,  J/.Z). 


Dioptric    Formula?    for    Combined    Cylindrical    Lenses. 


"This  little  brochure  solves  the  prob- 
lem of  combined  cylinders  in  all  its  as- 
pects, -and  in  a  manner  simple  enough  for 
the  comprehension  of  the  average  student 
of  ophthalmology.  The  author  is  to  be 
congratulated  upon  the  success  that  has 
crowned  his  labors,  for  nowhere  is  there 
to  be  found  so  simple  and  yet  so  complete 
an  explanation  as  is  contained  in  these 
pages." — Archives  of  Ophthalmology,  ed- 
ited ly  H.  Knapp,  M.D. 


"This  exhaustive  work  of  Mr.  Prentice 
is  a  solution  of  one  of  the  most  difficult 
problems  in  ophthalmological  optics. 
Thanks  are  due  to  Mr.  Prentice  for  the 
excellent  manner  in  which  he  has  eluci- 
dated i  subject  which  has  not  hitherto 
been  satisfactorily  explained." — The  Oph- 
thalmic Review,  London. 


The  book  contains  1 10  Original  Diagrams.     Bound  in  cloth. 
Price  $2.00 


Published  by 

THE  KEYSTONE  PUBLISHING  CO. 
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OPTOMETRIC  RECORD  BOOK 


A  record-book,  wherein  to  record  optometric  examina- 
tions, is  an  indispensable  adjunct  of  an  optician's  outfit. 

The  Keystone  Optometric  Record-book  was  specially  pre- 
pared for  this  purpose.  It  excels  all  others  in  being  not  only  a 
record-book,  but  an  invaluable  guide  in  examination. 

The  book  contains  two  hundred  record  forms  with  printed 
headings,  suggesting,  in  the  proper  order,  the  course  of  ex- 
amination that  should  be  pursued  to  obtain  most  accurate  re- 
sults. 

Each  book  has  an  index,  which  enables  the  optician  to 
refer  instantly  to  the  case  of  any  particular  patient. 

The  Keystone  Record-book  diminishes  the  time  and 
labor  required  for  examinations,  obviates  possible  oversights 
from  carelessness,  and  assures  a  systematic  and  thorough  ex- 
amination of  the  eye,  as  well  as  furnishes  a  permanent  record 
of  all  examinations. 

Sent  postpaid  on  receipt  of  $2.00 


Published  by 

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New  and  Enlarged  Edition  of 

STATE  BOARD 
EXAMINATIONS 

QUESTIONS  and  ANSWERS 


Compiled  by  C.  Henry  Brown,  M.  D. 
Author  of  "Clinics  in  Optometry,"  Etc. 


COMPILED  especially  for  the  student 
who  wishes  to  quiz  himself  before  taking 
the  State  Board  Examination  or  the 
practicing  Optometrist  who  desires  to  "brush 
up"  on  the  subjects  of  his  profession  and  find 
his  "weak  spots." 

Twice  the  size  of  the  old  edition.  This  new 
book  contains  1000  questions  asked  in  State 
Board  Examinations,  together  with  concise 
answers.  Eleven  chapters  covering  practi- 
cally every  question  asked  by  State  Boards. 

Bound  in  Silk  Cloth 

Price  $3.00 
Place  your  order  at  once 


Published  by 

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